The Project Gutenberg eBook, Zoology: The Science of Animal Life, by Ernest Ingersoll

Popular Science Library

EDITOR-IN-CHIEF

GARRETT P. SERVISS

AUTHORS

WILLIAM J. MILLER HIPPOLYTE GRUENER A. RUSSELL BOND
D. W. HERING LOOMIS HAVEMEYER ERNEST G. MARTIN
ARTHUR SELWYN-BROWN ROBERT CHENAULT GIVLER
ERNEST INGERSOLL WILFRED MASON BARTON
WILLIAM B. SCOTT ERNEST J. STREUBEL
NORMAN TAYLOR DAVID TODD
CHARLES FITZHUGH TALMAN
ROBIN BEACH

ARRANGED IN SIXTEEN VOLUMES
WITH A HISTORY OF SCIENCE, GLOSSARIES
AND A GENERAL INDEX

ILLUSTRATED

VOLUME TWELVE

P. F. COLLIER & SON COMPANY
NEW YORK

Copyright 1922
By P. F. Collier & Son Company
MANUFACTURED IN U. S. A.

ZOÖLOGY
The Science of Animal Life

BY

ERNEST INGERSOLL

P. F. COLLIER & SON COMPANY
NEW YORK

[PREFACE]

In this volume, occupying the place in the series assigned to the subject "Zoölogy," the writer was called upon to survey the whole range of animal life on the globe, and to keep in view the fact that these books were to be a library of science. The casual reader, with no particular interest in natural history, seeks in such a book little more than stories of animal life thought of mainly as "big game," with an appetite for the adventurous and wonderful. But beasts and birds and snakes, although they number in the aggregate thousands of kinds, are but few compared with the almost innumerable hosts of the lower orders of animal life that dwell in the wildernesses of the world, or throng in the seas, or hover about us in the air; yet they are a part of the zoölogy of the globe, and a most important part. Although they may rarely have the picturesque interest that attaches to the vertebrate groups, they exhibit great beauty in many cases, and are the foundation on which the others rest, for they furnish the food on which the more highly organized creatures subsist. To the student this lower half is often more attractive than the upper half; and the history and philosophy of animal life could not be understood unless it was fully considered. The author has therefore devoted a proportionate space to the lower orders, at the expense of detailed descriptions of birds and beasts, knowing that these are easily accessible elsewhere. The arrangement of the matter in the volume is according to the latest results of critics of classification, and it illustrates, as well as any lineal arrangement can, the principle of the development of the higher classes from the inferior by a gradual evolution toward more and more complex forms. Space did not permit of much exposition of methods of development, as revealed by fossils; and the volume on Paleontology should be read in connection with this one.

Ernest Ingersoll.

CONTENTS

LIST OF ILLUSTRATIONS

[CHAPTER I]
HOW THE GLOBE WAS STOCKED WITH LIFE

Ever since man began to think in the connected way that follows self-consciousness, he has pondered, with a mixture of fear, reverence, and curiosity, on the nature of life and its origin. The world in which he found himself was a vast mystery which, very crudely at first, he sought to penetrate. All his paths of thought led him circling back to himself as the greatest mystery of all. He struggled with the problem for thousands of years, framing fanciful guessworks, erecting elaborate structures of logic on foundations of error, emotion, and presumption, fashioning beautiful fables and theories (and waging wars to compel other men to accept them), yet found no better solution than that life must be a gift from some unknown, perhaps unknowable, source. Even lately, learned philosophers, such as Helmholtz and Kelvin, supposed it brought to the earth (in germs) by meteorites—fragments of exploded planets that had borne life before they went to destruction; or, like Arrhenius, postulated an impalpable dust, or "panspermia," scattered through all space and borne from the atmosphere of one planet to another. But all such hypotheses only threw the question of origin one step further back.

Meanwhile, beginning a few hundred years ago, when greater privilege of inquiry became possible in a jealous society, naturalists had tried to attack the problem from a new angle. They asked themselves whether they might not, by intensive study of living things, find the quality of life itself, hoping that if that could be done the source of it might be disclosed. In their earnest work they constantly improved their methods and their instruments, and so penetrated deeper and deeper into the constitution of plants and animals, until at last they found the ultimate particle in the cell and discovered living things so simple that they consisted of one cell alone; but why that microscopic particle was alive, while the grain of crystal beside it, or the drop of water in which it swam, was not alive, remained unexplained.

Thereupon some of the naturalists fell back into the ranks of the speculative and religious persons who were content to believe the endowment of the world with life an act of a Divine Creator—something above and outside of nature as otherwise manifested; others asserted an equivalent but more materialistic doctrine that they styled "spontaneous generation," which presently was shown to be untrue, at least in the way they formulated it; and a third group confessed that they did not know whence life came, nor were they much concerned to know.

LIFE CONSTANTLY ORIGINATED BY NATURE'S PRODUCTIVE ENERGY

This quest having failed—although it had taught much by the way—the chemists, who had been making marvelous discoveries in the inorganic lifeless half of nature, undertook a far more serious exploration of the organic living half. You have interpreted very fully, they told the naturalists, the forms, and structure, and functions of organisms, but can get no further; now let us chemists try whether we cannot find the principle of life by analyzing the substance of living things.

Profiting by their experience, they turned to the colloids in hope of a clue. A colloid is a substance that shows no power of crystallization, and is composed of molecules united by their own affinity, and not by atomic affinity. They have a gelatinlike nature or composition, although varying greatly in chemical composition and general character. They differ widely in stability, for instance, some being easily upset by a change in conditions; and this peculiarity is of great importance in relation to the phenomena of life, for colloids enter largely into the composition of all living bodies, but always in a delicately balanced union with crystalloids. "The colloid is in fact," declared Thomas Graham, who first investigated its properties, "a dynamical state of matter; the crystalloid being the statical condition. The colloid possesses Energia. It may be looked upon as the probable primary source of the force appearing in the phenomena of vitality."

Now, many of the properties of inorganic colloids approximate those found in living structures, which appear to be "alive" by reason of the conversion of the energy of the sunlight into the chemical energy of their constituent (organic) colloids. The agent in this conversion is the green substance chlorophyll in the cell or cells of the plant; and, directly or indirectly, all the energy in living things arises from this one source, transmuted by this one transformer. Yet chlorophyll is far too complex a substance to arise as a first step from inorganic matter, even where conditions are suitable for life to appear; and the spontaneous production of such a thing as a bacterium would not solve the problem, for the new-born cell would have no organic food, and must at once perish. In an utterly lifeless planet inorganic colloids must first develop, and in time one of these must begin to evolve not a living cell, or anything so complex as a bacillus, but something in the way of a molecule holding a higher store of chemical energy than anything before it. Later such colloids, perhaps uniting with others, would begin to condense and form more complex organic molecules, and finally effect unions with crystalloids. Thus would organic complexity gradually be led up to, chlorophyll brought into being, and life appear. One of the foremost of the biochemists, Prof. Benjamin Moore, of the University of Liverpool, has summed this up picturesquely:

"It was no fortuitous combination of chances, and no cosmic dust, which brought life to the womb of our ancient Mother Earth in the far-distant Paleozoic ages, but a well-regulated orderly development, which comes to every mother earth in the universe in the maturity of her creation when the conditions arrive within the suitable limits. Given the presence of matter and energy forms under the proper conditions, life must come inevitably.... If this view be the true one, there must exist a whole world of living creatures which the microscope has never shown us, leading up to bacteria and the protozoa. The brink of life lies ... away down among the colloids, and the beginning of life was not a fortuitous event occurring millions of years ago and never again repeated, but one which in its primordial stages keeps on repeating itself all the time and in our generation. So that, if all intelligent creatures were by some holocaust destroyed, up out of the depths in process of millions of years intelligent beings would once more emerge."

That is to say, life arose through a recombination of forces preexisting in the cosmos, and the fact was but a step in the evolutionary process. "Such evolution," the American biologist, Henry Fairfield Osborn, declares with emphasis, "is essentially constructive, and ... is continually giving birth to an infinite variety of new forms and functions which never appeared in the universe before. It is a continuous creation or creative evolution. Although this creative power is something new derived from the old, it presents the first of the numerous contrasts between the living and the lifeless world."

LIFE'S BORDERLAND

Although in some respects a deceptive resemblance may appear between the living and the nonliving, the distinction is definite. Living bodies, plant or animal, are made up of protoplasm, which, although mineral in substance, consists of a combination never found in the mineral kingdom. It gives to the body containing it the power of growth, and this growth is by additions from within. Minerals may increase in size, but only by additions from without. The prime characteristics of living organisms is that they reproduce their kind, given favorable conditions. Minerals never do so. A correlative of life and growth is death, but minerals never die. In the course of its career every animal or plant, in proportion to its need or the degree of complexity of its organs, develops within itself characteristic compounds, such as albumin, gluten, starch, cellulose, fat and other chemical results, not a trace of any of which is to be found in rocks or soil, or in the water or in the air. No distinction in nature is so absolute as that between the inorganic and the organic realms, the nonliving and living things, so far as our senses can perceive them.

When, however, we consider the two prime divisions of the living world—animal and vegetable—so diverse in their higher developments, we find them springing from the same base in a single cell of almost structureless protoplasm, and so alike in this simplest form as to be in some cases indistinguishable—mere drops of living matter whose functions are so limited that they present no discriminative characteristics. Indeed, marking a definite boundary between animals and plants may be difficult in cases much higher in the scale than these primitive globules of protoplasm.

A fundamental distinction between plants and animals as we now know them is the exclusive possession by plants of the green substance chlorophyll, by the presence of which their food is transformed under the influence of sunlight into vital energy in a manner essentially different from that by which animals assimilate their substance. Chlorophyll is a complex, nitrogenous, colloidal substance, produced by and always associated with, protoplasm, and related to the coloring matter of the blood of animals. It is restricted to plants, and usually resides only in definite portions of the cell; yet we have good reason for believing, as Prof. William F. Ganong tells us, that our present green plants were preceded in time by a colorless kind of the utmost simplicity, and without chlorophyll, which yet could make their own food from carbon dioxide and water by using the energy of chemical oxidation of soil-minerals in place of sunlight. "We have precisely such chemosynthetic organisms, a kind of soil bacteria, still living on the earth at this day; and they are doubtless the lineal descendants of the ancient forms, which probably lived in the mud of shallow seas that may be full of them yet." These ancient chemosynthetic organisms were neither animal nor plant, but both and between. They must have expanded, varied, evolved, thus originating a great many branches, most of which perished.

Now, from this biochemical borderland of life, let us turn our attention to the living world as we know it to-day, or as preserved for us in the "record of the rocks," pausing only to fix well in our minds the main distinctions between animals and plants. Plants have no special organs for digestion or circulation, nor any nervous system. Most plants absorb inorganic food, such as water, carbonic acid gas, nitrate of ammonia, phosphates, silica, etc. No animal swallows any of these minerals as food. On the other hand, plants manufacture from such materials the food on which animals exist, by the production and storage in their tissues of starch, sugar, and nitrogenous substances. The two kingdoms supplement one another. They are mutually dependent, and probably originated simultaneously.

[CHAPTER II]
THE SEA A VAST AQUARIUM

No results of investigation in natural history have been more amazing than those that show the marvelous richness of the sea in plant and animal life—not merely at its warm margin, but far out in the centre of what the ancients used to call "the desert of waters"; not only at its surface, but in its profoundest depths, and under the polar ice as well as amid the tropics. Sea populations differ somewhat according to situation, those of the shallow shore lines, which are of the "littoral" fauna, differ largely from those living in the open sea and belonging to the "pelagic" fauna, and there are surface swimmers, and others confined to the abysses; but virtually every class and subdivision in the animal kingdom is represented in greater or less variety in the zoölogy of the ocean. The list stretches from the merest monads to the huge sharks and still bigger whales.

This multitude and diversity of animal life is possible in the sea because of an even greater plenitude of plants there, which furnish a never-failing food resource. Bacteria and blue-green algæ are at the base of this. Bacteria exist in all seas, as in all soils, and the fertility of nature above ground and under water depends on these microscopic organisms, whose numbers in the ocean are as incalculable as the grains of sand on its brink. In equal multitude are the diatoms, unicellular algæ with flinty cases, by which the waves are sometimes discolored over broad areas; and millions of other green plants, living alone, or in chains, minute in size, but each a chemical laboratory converting the salt water they absorb into meals for the animals that swallow them—animals in most cases almost as small and simple as the things they eat, and themselves destined to be sucked into the mouth of something a little bigger, to be in turn a tidbit for a third hungry mouth, and so on to the broiled mackerel for our own breakfast.

THE PLANKTON AND ITS PHOSPHORESCENCE

The assemblage of plants and animals that together float or swim at or near the surface of the ocean (or other water), say within a layer of water one hundred fathoms thick, is scientifically called plankton of the sea. In the open ocean, the pelagic plankton is much alike all round the world of waters, although it varies a little in composition, and still more in relative abundance, being denser in temperate than in either tropical or polar latitudes; but nowhere is it absent. The "waste of waters" teems with life. The plankton of the shallow waters near continental shores, however, presents a decidedly different assemblage from the pelagic plankton.

In the pelagic plankton, single-celled animals of the groups called foraminifers and radiolarians are exceedingly prominent, and play an enormous part in the economy of the sea, although almost or quite microscopic in size. They are incased in chambered shells of lime or flint; and over vast areas in warm latitudes the ocean floor is so thickly covered with the dead shells of one kind that the mud is called globigerina ooze. They are the eaters of the microscopic plants, and themselves are food for a wide variety of hydroids and jellyfish, large and small, whose silvery forms are often visible to the voyager, and which are mostly responsible for the pale stars of phosphorescence that shine about his prow and glorify his wake in dark nights. The queen of these far swimmers is the radiant Portuguese man-of-war. In the night a dragging fine-meshed net will capture more than by day of the plankton, because many little creatures that in daylight sink to considerable depths come to the surface at night.

Rising a step to the worms, we find them comparatively rare, but one kind of marine flatworm that abounds in midocean is rose-red and several inches long. Much more numerous is another flatworm, Sagitta, "which along with copepoda, salpæ, pteropoda and radiolaria, everywhere constitute the bulk of the small pelagic organisms" captured by towing nets. Like almost all of these usually defenseless creatures they are perfectly transparent, but some of them depart from the rule of pale blue in tint and shine in bright red. A longer step takes us to the Crustacea, represented in the pelagic plankton by queer little shrimplike forms that in countless hosts of individuals play a part in the ocean comparable to that of insects on land. The copepods are the most numerous probably—little things only a fraction of an inch in length, but amazingly abundant, and the principal users of plant food. Their relatives, the little ostracods, have similar habits, and are noted for their intense phosphorescence. Haeckel relates that on his way to Ceylon he saw the entire sea like a twinkling ocean of light, and his microscope showed him that it was made by throngs of ostracods, with some jellyfishes, salpæ and worms. Crustaceans of higher rank abound also. In northern waters species of Schizopoda, small, transparent prawns with red spots around the mouth and big, black eyes, swarm in enormous numbers, and are known to the fishermen as "kril."

An important part of the pelagic plankton consists of certain small mollusks; and "as regards abundance of individuals few groups of pelagic animals can compare with the winged snails, or Pteropoda." These are minute, rapidly swimming creatures with thin, glassy shells, and in some parts of the warmer oceans these discarded shells are so numerous on the bottom that they give the name pteropod ooze to the mud. One kind (Limacina), with a coiled shell about the size of a pinhead, which abounds in the north Atlantic, is much feared by the Norwegian fishermen because they very often spoil the herring that feed on them. Another kind (Clione), looking somewhat like a reddish butterfly an inch or so long, swims in shoals in the icy seas of the far North, and is known as "whales' food." Some larger mollusks, of which the beautiful purple Ianthina is most conspicuous, live among the vast patches of floating seaweed in the Sargasso Sea.

Great numbers and variety of tunicates or ascidians and their larvæ are taken in the surface nets of the sea naturalists, among them the salpæ—free-swimming, barrel-shaped, transparent animals well known to all seafaring people, and often seen crowding the surface of the ocean. One genus of them is Pyrosoma, which has from the earliest days excited the interest of mankind, mainly on account of the strong phosphorescent light emitted, the name, indeed, meaning "fire animal." These salpæ aggregate into colonies often several yards in length which glow like fiery serpents as they move sinuously on their way.

This property of luminosity, so widely possessed by marine animals, is one of the unsolved mysteries. It is called "phosphorescence," because it resembles the cold light given by phosphorus when undergoing slow oxidation, but phosphorus has nothing to do with the manifestation here, or in such insects as the firefly; nor is it owing to bacteria, as in the case of shining wood or decaying fish. What it really is no one knows, but it has, at least, been learned that in animals the power of emitting light is always attributable to certain structures of a glandular nature that secrete a slimy, luminous substance, or, rather, two substances, one luciferin and the other luciferase. When both together are exposed to seawater phosphorescent light results.

As a rule, the light organ is surrounded by a layer of black pigment that acts as a reflector, and often the light is projected through a transparent lens; and there is reason to believe that in the case of the higher animals, such as deep-sea fishes and squids, the rays may be thrown when and where the creature desires, as a man handles an electric flashlight. But for what purpose? Is it to illuminate the surrounding water so as to perceive, or to attract prey, or is it to avoid foes? A learned oceanographer replies that no one certainly knows. "At all events," he concludes, "the answers would probably tend to show that the many different kinds of light organs serve different purposes."

PRISONERS IN THE DARK AND ICY DEPTHS

So much for the surface population of the ocean—the plankton layer is regarded as a hundred fathoms thick. We have considered only that over the mid-oceanic depths, but that of the shallow margins is different simply in the absence of some purely pelagic creatures, and in the presence of vast hordes of eggs and larvæ of the animals rooted in the sand or attached to the rocks and weeds from high-water mark down to a comparatively short distance below low-water mark. These I shall speak of more completely hereafter.

Before that, however, I want to say a few words in regard to the extraordinary inhabitants of the ocean's depths—depths which in some places exceed the elevation of the highest mountains on the land.

The conditions under which animal life exists there are vastly different from those at the surface, and it is not surprising to find these creatures of an extraordinary character. The pressure exerted by water on anything lowered into it increases at a rapid rate as the object sinks, so that at a depth of only 500 fathoms it equals about 100 times the pressure at the surface. This contributes to the density of underlying waters; the saltiness of the sea also adds to the water's density, but this decreases slightly from the surface downward. More important than density in its effect on living things is temperature. In the Sargasso Sea in summer the water at the surface will indicate about 52 degrees F., and at 100 fathoms of depth 48 degrees, below which it diminishes slowly to a little below the freezing point—32 degrees F. The water below a few hundred fathoms may therefore be regarded as a series of layers measured by degrees of density, temperature, etc., and this means a series of biological strata in each of which the denizens are more or less limited by unfavorable conditions above and below them.

A fourth factor conditioning deep-sea life is that of light. The sunlight penetrates to a much greater distance than was formerly believed; and experiments with photographic plates show that the blue rays may sink as far as 800 fathoms, but the red rays go much less down. Below that glimmer is absolute darkness, illuminated only by the phosphorescent glow of the lanterns carried by the animals moving about in that Stygian and icy abode—which would seem to us the most dreadful fate to which any creature on the globe is born.

It has been said that the ocean depths seem to be divided into horizontal zones, certain groups of animals being confined, when adults, within limits of depth determined by conditions suitable to them, one zone above the other. Practically, however, these intermediate life-zones can hardly be defined, and vary in different seas, and under changing conditions, as of season, and so forth. Animals taken only by deep hauls of the nets within the tropics, for instance, may be captured in cooler latitudes near the surface; furthermore, the vertical distribution of fishes, as a class, may differ from that of crustaceans as a class. Nevertheless it is true in general that many sorts of pelagic animals dwell at intermediate depths, from which, when they have become mature, they cannot either rise or descend any great distance. Among them are representatives of all the classes of marine life.

Let us now consider the creatures of the lowest level—those abysmal depths where eternal cold, stillness, darkness, and equability unite to make an environment so forbidding that human imagination would refuse to people it with living beings; yet where life and strife do actually exist, although by no means uniformly distributed. We know most about it as it exists in the bed of the north Atlantic.

The real bottom animals are mainly fixed—sponges, hydroids, sea anemones, bryozoans, brittle-stars, crinoids, brachiopods, holothurians, worms and mollusks. They are nowhere numerous remote from a shore, and below 2,500 fathoms are very scarce, to judge by the results of dredging. Their food comes wholly from the surface, apparently, some catching it as it falls and others sucking it out of the ooze. Moving about among these, and feeding on them, is a scanty population of snails, squids, crabs, and fishes, making their living upon or close to the bottom; and a larger and more varied company of relatives swim in the water above them up to, say, the 2,000-fathoms line. All these are of forms different in many respects from kindred species at or near the surface; and some brought up by the deep-sea dredge can hardly be distinguished from fossils entombed in the oldest fossiliferous rocks—so unchangeable is the environment in which their race has been propagated for perhaps fifty millions of years.

Through these dark abysses swim fishes with extraordinary and grotesque adaptations to their conditions. All are small, rarely six inches long, often less than an inch, yet armed to the teeth. This is especially true of the families Stomiatidæ and Sternoptychidæ, in which one finds fishes of the queerest shape, with big heads and a savage array of long sharp teeth. All are voracious, for food is scant and must be fought for; and some, as Chiasmodus, have mouths so capacious that they often swallow fishes larger than themselves, when their stretched stomachs hang beneath their slender bodies like the yolk sacs of newly born trout. All are dark in color, brown, blue or violet marking the abyssal species. Some of them have light-giving organs; and this was formerly regarded as a peculiar possession of deep-sea fishes, enabling them to see their prey in the gloom of their habitat, but it is now known that light-giving organs are especially characteristic of pelagic fishes of the region between the surface and 250 fathoms of depth. It must be remembered, however, that the sedentary invertebrates of the bottom glow with phosphorescence.

This outline of a vast body of information shows that the waters of the oceans are everywhere inhabited, to their uttermost deeps, by living beings; that these are adapted to various circumstances, and so form faunas of local extent and character; and that probably the sea derived its wealth of population—at least all that part superior to the monads—from the land, beginning with the earliest dawn of life on the globe.

[CHAPTER III]
A CHAPTER OF FOUNDATIONS

THE NATURAL BASIS OF CLASSIFICATION

I mentioned in my introductory chapter that the simplest form of animal was one whose whole being was contained within a single envelope, or "skin," called a cell. Such a cell contains nothing but that strange primitive life-substance named protoplasm, condensed at one point into a nucleus, and it is precisely of such cells that the bodies of all the animals we commonly know are made up; nevertheless an immense variety of creatures still exists, especially in the plankton of the sea, that, like those at the dawn of life, consist of one cell alone. Here then we stand at the first grand division of the animal kingdom:

A. Animals consisting of a single cell—Protozoa.

B. Animals composed of an aggregation of cells—Metazoa.

This distinction, you see, is one of structure, as must be all the subdivisions that follow, if they are to be natural; and it is the clearest possible illustration of what we mean in zoölogy when we speak of "lower" and "higher" rank, for it is evident that it is a step upward, an advance from utter simplicity to greater and greater complexity, to proceed from a single-celled, all but helpless animalcule to one composed of many cells, with so vast a division of labor and extensive power of action as belong to such a combination of forces.

I do not propose to describe the Protozoa, because both of lack of space and lack of popular interest; anyone may learn about them in any good zoölogical textbook. But I do want to mention one very important point, on account of its bearing on the history of the higher animals. The protozoans reproduce their kind by simply splitting into two individuals, and these again split into another two, and so on; the process is called "fission." There comes a time, however, when the ability to do this ceases, and the protozoans of this strain will die out unless one or more of them meets with the same kind of animalcule, and the two "conjugate," or merge into one another, thus renewing their power to go on dividing.

Turning now to the Metazoa, or animals in general, we may say that they are flexible and usually motile beings, needing a supply of solid food which they convert by digestion into a fluid form, and then diffuse through their tissues. This accounts for the fact that all animals consist essentially of a tube, which in the simpler forms is very apparent. This typical tube consists of at least two layers—an outer, protective, and sensitive coat (ectoderm), and an inner, digestive one (endoderm). This two-layered condition is the limit for a few fresh-water and a vast number of marine animals therefore called "cœlenterata," of which the jellyfish and corals are examples. The two coats are separated, and at the same time connected, by a greater or less amount of a jellylike filling called the "mesenchyme." Into this intermediate mesenchyme both ectoderm and endoderm bud off cells which have certain functions—that is, they circulate the digested food, perform the creeping movements when such occur, expel the waste of the body, and most important of all, provide the germ cells by which the race is perpetuated.

Now in animals superior to the jellyfishes and the flatworms, the mesenchyme is replaced by a definite hollow tissue that produces a more efficient system of muscular, excretory, and reproductive organs. This hollow tissue is the "cœlom," and in the most advanced animals, such as the chordates, "the cœlom and its products are of the greatest importance, for they give rise to the vertebræ and the muscles, and in so doing mold the shape of the fish, amphibian, reptile, bird, and mammal."

In this brief sketch of some broad distinctions among the masses of animals we have a hint of the basis of their classification.

ANIMAL LIFE IN ORDERLY ARRANGEMENT

Classification is really only a sorting out of things into groups of the same kind. It may be artificial, according to fancy or convenience, or it may be by discovery of nature's inevitable development. It has been done crudely ever since men began to show curiosity about the things around them. They spoke of animals of the land, of the water, and of the air; of those that lived on vegetable fare as different from the flesh eaters; and in a more particular way they recognized various obviously like and unlike groups within the larger ones. All these distinctions were made on external appearance or behavior, and closer observation presently showed bad combinations, such as placing bats with birds simply because both flew, or whales with fish because both lived in water. Slowly it became evident that the only proper way to classify animals was by putting together those of like structure, and this could be accomplished only by intense comparative study of the interior anatomy of their bodies. Even here, however, progress was limited until the great light from the idea of organic evolution fell on biological science, by which it was perceived that the true criterion by which the proper place of any animal could be determined was its line of descent—a matter wherein the student of fossils could render, and has rendered, vast assistance. In other words a real, natural classification is according to ancestry, just as human relatives are grouped into families according to their known descent from the same forefather.

In this evolutionary light zoölogists have now perfected, at least in respect to its larger divisions, a classification of the animal kingdom which is generally accepted, and is followed in this book. It proceeds, reading downward, from the simpler and older forms of animal life to the more complex and more recent forms.

As to the names and relative order, or rank, of the subdivisions that we shall have occasion to mention, a few words are desirable. The only real fact is the individual animal. A collection of these so similar that they cannot be divided, and which will interbreed, but usually are sterile as to other animals, is termed a species. A number of species closely similar are bracketed together as a genus (plural genera), and this done, every individual is given a double name, as Felis leo to the lion, the first part of which indicates its genus, and is called its "generic" name, and the second indicates its species, and is called its "specific" name. This "scientific name" is given in Latin (or Latinized Greek) so that it may be unmistakably understood in all parts of the world, for a local name in one language would mean nothing to a student speaking some other language, or perhaps speaking the same language in another country; thus the name "robin" is applied to half a dozen very different birds in separate parts of the English-speaking world, and endless confusion would result were not each animal labeled in a language understood by everybody; and this must be a dead language, so that the significance of the terms applied shall not vary in place or time.

Several similar genera may form a family; families that agree in essential characteristics are united as orders; orders are grouped into classes; and finally like classes are assembled into a phylum (Greek, "a leaf": plural phyla), which is the largest division except the primary distinction of Protozoa and Metazoa.

[CHAPTER IV]
THE HUMBLEST OF ANIMALS—SPONGES

At the foot of the arrangement of phyla in the metazoa stand the Porifera, or sponges, fixed, plantlike, queerly shaped beings living in the sea, except one family in fresh waters, and abundant in all the warmer parts of the world on rocky bottoms. Whatever its size or shape, a live sponge (of which the commercial article is the more or less perfect skeleton) is coated with a thin fleshy membrane perforated by minute "inhalant pores" and larger holes termed "oscula," or mouths. Through the inhalant pores the sea water, with its burden of microscopic food, enters one of many spaces beneath the surface from which incurrent canals penetrate the interior of the sponge, constantly branching and growing smaller until lost to sight. The fine tips communicate with small cavities lined with cells that are fitted to seize and assimilate the nourishment brought them by the water. From these rudimentary stomachs go similar excurrent ducts that unite near the surface into trunk canals that carry out the used water and waste products. This system of circulation, bringing nutrient water strained through the pores, and expelling it forcibly after it has been cleared of food value, is kept in motion, with occasional periods of rest, by the action of "flagellate cells" that line certain tracts in the canals. These are elongated cells from which project whiplashlike filaments, one to each cell, whose movements in concert "resemble those which a very supple fishing rod is made to undergo in the act of casting a long line"—the movement being much swifter from without inward.

Beneath the outer skin, and all among the canals and cavities, is a filling of gelatinous materials, largely protoplasm, in which are formed great numbers of variously branched and strengthening spicules, of limy material in one group, and in others of a flinty or glassy nature, or in the absence of these, a network of "spongin," such as forms the skeleton of our common washing sponges. Spongin is a substance allied to silk in chemical composition, and the threads are felted together in such a way as to form a firm, yet elastic structure. "In some Noncalcarea, which are devoid of spicules, the place of these is taken by foreign bodies—shells of Radiolaria, grains of sand, or spicules from other sponges. In others again, such as the Venus's flower basket (Euplectella), the glass-rope sponge (Hyalonema), and others, the skeleton consists throughout of siliceous spicules bound together by a siliceous cement."

Sponges are reproduced both by budding in some form, which is an asexual way, and by the sexual method of eggs and male cells; these are formed in the same sponge, but rarely at the same time, and the early stages of development are passed in a brood-cell within the body of the parent sponge. Finally, the embryo escapes through one of the outgoing canals, swims about awhile, becomes thimble-shaped, and settling down, fastens itself by the closed end to some patch of mud, a rock, dead shell or seaweed, closes the open end of the "thimble," and proceeds to grow.

Sponges do not appear to be eaten by fishes or anything else. Countless lower animals, such as marine worms, mollusks, and so forth, burrow into them, however, in search of shelter; and in reversal, certain small sponges, such as the cliona of our shores, burrow into the shells of mollusks, which explains the honeycombed appearance of many of the shells picked up on the beach. Sponges have a large part in that very interesting and widespread phase of marine life called "commensalism," in which two animals become intimately associated in a mutually beneficial way, and are thus spoken of as messmates. Some kinds of sponge are never found growing except on the backs or legs of certain crabs; the sponge conceals and protects the crab, while itself benefits by being carried from place to place, with constantly new changes of fresh water and food. This sort of partnership occurs in many different groups of marine animals.

The capture and preparation of sponges for market employ thousands of men and boats in the eastern Mediterranean, whence the best are derived, and in the West Indies and Gulf of Mexico, where the sponges are of a coarser kind, and are gathered and prepared by rougher methods. They are taken commercially also in other seas, and frequently dredged from vast depths.

[CHAPTER V]
FLOWERS OF THE SEA

JELLYFISHES, NAMESAKES OF THE FABLED MEDUSA

The type and simplest form of that great division of aquatic, and almost exclusively marine, animals constituting the phylum Cœlenterata, is the polyp. It consists of a soft-skinned body, typically cup-shaped, containing a baglike digestive cavity, or primitive stomach, open at the top, and surrounded by the soft mesenchyme. The open upper end is the mouth, which is usually encircled by few or many tentacles—hollow outgrowths from the wall of the tubular gullet. Currents of water are drawn in by waving cilia at one end of the slitlike mouth, and pass out as waste at the other side; they bring food and oxygen from which nourishment is absorbed by the cells of the wall of the stomach (endoderm). Certain outgrowths within the mesenchyme act as feeble muscles for lengthening and shortening the body and tentacles; but there are no blood vessels or excretory organs.

Most polyps are fixed on some support, but in many the young pass through a free, swimming stage before settling down for life. All cœlenterates, and these only, are provided with "stinging cells," the nature and importance of which will be explained presently.

The simplest class is that of the hydroids (Hydroida), the type of which is the fresh-water hydra, so-called because, like the Hydra of ancient myth, when it is cut to pieces each part will grow into a new animal. It lives in ponds and pools of stagnant water, and is so small that a magnifying glass is necessary to study it, especially in the case of the green one of our two common American species—the other is brown. Indeed, similar hydroids of salt water are often taken and dried by unscientific collectors under the impression that they are feathery seaweeds. It is stalklike in shape, has long tentacles which always turn toward the greatest light, influenced like certain plants by heliotropism, and feeds on minute crustaceans and other minute organisms. Sometimes hydras are so abundant as to form a velvety surface in warm pools. The sexes are combined in the same individual, and the embryo forms within the body, then protrudes as a bud, which finally breaks away and after a time sinks, attaches itself at the base to some support, and grows into a perfect hydra. When quiescent or alarmed the tentacles are withdrawn, and the whole animal shrinks into a little lump.

Such is the general natural history of the group; but the oceanic hydroids have developed a vast variety of forms, and, with increased breadth of life, have added many interesting features and habits. Many of them are single, rooted in mud, or upon seaweeds, rocks or shellfish both dead and alive, and look like flowers of lovely tints; and they reproduce by putting forth separate reproductive parts, called "zooids," of various kinds. Others are in colonies that spread by extensions of the base from which arise other hydroids until a bunch of them are growing side by side; but these groups consist of hydroids differentiated into separate functions, for some devote themselves to capturing food which nourishes all, through the common base, while others produce the buds and eggs by which the colony is increased.

The most remarkable of these processes of reproduction is that which is represented by the jellyfishes so abundant in all seas, and so beautiful either when seen floating along just at the surface of the summer sea, or when at night they glow with phosphorescence like silvery, greenish rockets in the dark waves. Sometimes they occur in enormous "schools"—as we say of fish—all of one kind, filling the water thickly as far as one can see, and now and then in late summer are cast on the beach in long windrows. They range in size from a pinhead to ten or twelve feet in diameter. So big a Cyanea would probably weigh fifty pounds, but after a thorough drying would yield only a few ounces of semisolid matter, 99 per cent of the creature being water absorbed in its spongy tissues. Some are egg-shaped, others like a bell with a long clapper, but the ordinary form is that of an open umbrella, usually fringed about the edge with tentacles, sometimes short and fine, sometimes few and long, again a crowded circle of long snaky appendages. These elastic hanging tentacles are the means by which the medusa (as such a jellyfish is appropriately termed in science) captures its food, which consists not only of the minute things swarming in the plankton, but of other cœlenterates, small crustacea, fishes, anything in fact that it can entangle in its sticky net and sting to death. Every one of the filmy tentacles is thickly studded with microscopic cells (cnidocells) covered by a mere film, and having a spinelike trigger projecting from it. If this trigger is touched, or the film broken, out springs the coiled thread dart which is barbed and carries into the wound it makes a poison that benumbs. Thousands of these microscopic darts may prick the skin of a captive, and paralyze its strength—as it does that of a man who gets caught naked in the trailing net of one of the great northern medusæ. Being thus captured, the prey is drawn up to the mouth, which opens in the center of the under side of the umbrella float.

At intervals around the margin of the umbrella are small organs by which, it is believed, the creature maintains a sense of balance and direction, and perhaps of temperature or light, or both; for many medusæ sink out of sight by day and come to the surface at night; and when the sea is rough they descend to quiet depths. Thus they have the power not only to move ahead by the alternate contraction and dilatation of the disk, but to so alter their specific gravity as to sink or rise at will. They thus show the rudiments of both a muscular and a nervous system.

Very interesting, and often of great beauty, are the free-swimming, colonial, hydroid polyps called siphonophores. On a long stem or string are arranged, at the top, a bulb filled with gas or air, as a float, then a series of swimming bells whose pulsations carry the colony about, beneath which are various polyps and tentaclelike appendages, some to gather food, whose digested products circulate through the whole colony, others performing reproductive functions. The variety of form is considerable; and one of the most peculiar, and the only siphonophore familiar to most persons, is the exquisite Portuguese man-of-war, whose prismatically tinted bulb, as big as one's fist, is commonly met with in the Gulf Stream in the North Atlantic, and often is seen in great flocks in the tropics, bobbing on the surface of the waves in calm weather. Beneath that bulb trails a long tuft of tentacles and zooids, performing various functions, and so foreshadowing the division of labor that in the higher animals is effected by the different limbs and organs.

SEA ANEMONES, CORALS, AND SEA FANS

Sea anemones are simply large polyps of more complicated structure than the hydroid polyps. Instead of a simple, baglike, enteric cavity, the slitlike mouth admits food into a flattened gullet which leads to an enlarged digestive cavity. The gullet does not hang free, but is joined to the outer wall of the body by a series of radiating partitions, between which shorter ones extend from the inner surface of the ectoderm; and below the gullet the stomach wall extends in lobes between these partitions, through which holes permit the nutritive juices to circulate throughout the whole body. The whole upper surface of the polyp is covered by short tentacles arranged in circles. A current of water, induced by waving cilia, is constantly flowing in at one corner of the mouth and out at the other, supplying the animal with oxygen and a certain amount of minute food, and carrying off waste; but the anemones capture by means of their tentacles small fishes, mollusks and everything that can be caught and swallowed. As some anemones exceed a foot in diameter, large and powerful prey may sometimes be taken. It is interesting to note that anemones distinguish very quickly between what is good to eat and what is not. Most of them are sitting near shore on rocks or in tide pools, or are clinging to the larger seaweeds or clustered on the supports of wharves where the waves and tidal currents are continually washing about them, often with much violence, and dashing against them strands of weed or the small wreckage always floating in such a place. None of this is seized, or at least is not swallowed; but whether we are to conclude that this choice is made by intelligence, or only by chemical perception is a matter for study. When harm threatens, or when they crave rest, they withdraw all their gorgeous tentacles, infold them within their mouth, and shrink down into roundish gray lumps that attract neither the eye nor the appetite of any marauder.

The coral polyps differ from anemones only in details of structure that we need not consider, except to note the striking difference that here the base and the radiating partitions instead of being membranous secrete a firm skeleton either of lime or of the horny material termed chitin. The flesh overflows the walls, folding down from the top, so that the skeleton becomes really internal, although naked at the broad base. Some of the tropical stony corals are like big anemones, several inches across; and it is only when they infold all their richly colored tentacles and become a dull and shapeless lump that their stony cup is revealed. These are solitary, and form loosely lying corals, like that called the "mushroom." New ones are produced by the parent throwing off buds which for a time remain attached by a stalk, but finally fall off and settle down to grow—a process that may go on for a score of years. In the case of the huge coral masses called madrepores the buds remain attached to the parent. If they spread out naturally, W. Saville Kent explains, they build up by accumulation the large rounded masses known as "brain" corals and "star" corals, which are most numerous on coastline reefs, or form the base of the outer barrier reef. On the other hand, where the budding is terminal, or oblique, branching, treelike growths result in "staghorn" and similar forms.

The coral animals do not alone construct the reefs. Stony hydroids (millepores), shells of all sorts of mollusks, limy sea mosses (Bryozoa), animalcules and diatoms and various algæ stiffened or cased with lime or flint, and blown sand, contribute to build them up, especially when they near the surface of the sea.

The distribution of reef-building corals is interesting. At present they are limited to about 35 degrees each side of the equator, but are irregularly distributed, owing mainly to differences of temperature in the water, which must not be colder than 68 degrees F. Hence they exist farther away from the equator in the path of warm ocean currents. The Gulf Stream accounts for the coral islands along the coast of Florida and in the Bermudas, which is their farthest point on the American coast; and the warmth of the water accounts for their extensive presence along the eastern coasts of Australia and Africa, when few exist on the western sides of these continents; similarly the western coasts of South and Central America are nearly free of coral banks. Other causes of limitation exist. For example, the noticeable absence of coral growth along the coast of South America is largely, if not altogether, owing to the fresh water and silt brought down by the great rivers there—both prejudicial to coral life.

Coral colonies increase and ultimately form banks wherever warm, pure sea water is constantly present, and not more than about 125 feet deep. Here, spreading and continually rising on the skeletons of dead generations, they form a long line close to the land called a "fringing" reef; and outside of this, beyond a space swept by the currents, may arise a second, still more flourishing bank, termed "barrier" reef. The great barrier reef that extends for 1,200 miles along the eastern coast of Australia—a vast chain of banks and islands—is an amazing example of what these minute animals can accomplish, given time; and geology can point to still more stupendous results of their work in the early history of the globe.

Very characteristic, in the great coral-growing region of the South Sea archipelago, is the ring-shaped island or "atoll," which incloses a quiet lagoon, usually with an open entrance. The reason for such a form has excited much discussion, one explanation being that its origin was about a small island that slowly subsided, the coral keeping pace in rising as the island sank, until finally the land disappeared; another that the circular reef arose from a submerged elevation, and when it came near the surface ceased to grow except on its outer border because it ceased to get suitable water and food, until after a time the central part died out, leaving a ring. Both explanations may be true of different situations.

When a reef comes near to the surface the branching coral is knocked to pieces by the waves, and there are added to this breakage shells and bones, calcareous seaweeds, and what not; and all this is ground into sand by the surf, washed high on the top of the ridge and manured by dead plants and animals, and by the droppings of birds, until finally a soil forms beyond the reach of the tides. Then, if it is in the far southern seas, a drifting coconut may lodge there and be rolled high enough to be left to strike its roots into the sand and begin the grove that by and by will make the islet attractive to men. The thick husk of the coconut resists harm from sea water, near which this palm prefers to grow in just such a sandy, shelly soil as the uprising reef affords. The nuts that so often fall into the surf or are carried out by rivers make long voyages without losing their vitality. Here, again, the situation of most coral islets in the course of currents is advantageous, for thus not only these nuts but other useful seeds and colonizing elements drift directly to their doors, as it were. Birds, wandering widely over the waters, espy the bit of land, and aid by their visits to increase its fertility and often add to its flora. Reefs near shore, especially in Florida and southward, become jungles of mangroves, which not only spring from floating seeds but send down from their branches sprouts that become rooted in the mud and spread the growth interminably. Such a "mangrove key" soon attracts an extensive population of plants and animals and speedily becomes a considerable island.

A great variety of corals, however, are not reef builders, and some species secrete little if any lime; these solitary relatives are found scattered all over the oceans, in deep water as well as shallow, wherever the bottom is suitable, and an immense amount of interesting information about them is to be found in books devoted to this beautiful group of animals.

The class includes two or three other orders of coral—polyps that grow in a solitary way or in groups, forming those elegant objects called sea fans, sea pens, and so forth, which can be referred to only briefly. One of these is the order Alcyonaria, in which some are soft-bodied, others are strengthened by a network of spicules. A very beautiful one is the "sea pen," which takes the shape of an ostrich plume; another is the strange mass of parallel tubes called organ-pipe coral; and some of them are very large, the great tree coral of the eastern Atlantic depths being sometimes as tall as a man, while it looks like a sturdy, leafless tree. As in all the others, however, it is covered by a living fleshy coat of protoplasmic substance studded with polyps whose gay colors and waving tentacles give it the appearance of being clothed with minute sessile blossoms. The best known of this group, probably, is the red coral of commerce, which is the scarlet, ivorylike interior stem of a branching alcyonarian colony. This coral has from the earliest time been cut into cameos by lapidaries, as well as used for making necklaces and other toilet ornaments.

[CHAPTER VI]
UNINVITED GUESTS

FLATWORMS, FLUKES AND TAPEWORMS

The phylum Platyhelminthes follows the cœlenterates in the ascending series of zoölogical classification, and includes a baneful company of creatures badly called "worms," which show none of the segmented or ringlike form of body that characterizes the true worms of the phylum Annulata to which we shall come presently. On the contrary, they are a group of small, soft-bodied, flattened animals, which first show that two-sided character, or bilateral symmetry, which has apparently been absent from all the groups we have studied hitherto, whose members are circular or globular in shape, and whose organs, in the adult, are arranged radiately.

The simplest are the planarians (Turbellaria), which live a free life, as a rule, although some are parasitic. They are little, thin, leaf-shaped creatures that creep on the bottom of ponds and even of deep lakes, or swim in the sea, and feed upon algæ and minute animals.

Similar to them in appearance are the flukes (Trematoda), of which the best known of a large variety is that which infests sheep. Most of the trematodes are parasitic.

The third class of flatworms is the Cestoda, the members of which are universally parasitic, and are known principally as "tapeworms" in reference to their form.

The phylum Nematothelminthes contains an assemblage of related worms, some marine, but mostly living in fresh waters or on land, which are eellike in form, very slender, and often have amazing length. The first and lowest class is that of the nematodes, of which the minute "vinegar eels" and "paste eels" are familiar examples. The remainder of the nematodes are parasitic, and many of them are dangerous parasites.

In an allied family and genus (Trichina) is placed one of the most dangerous of human parasites, the Trichina spiralis.

Here, too, comes that "hairworm" (Gordius), which most country folks call "hair eel" or "hair snake." Many assert with the most positive faith that if you will soak a horsehair in water it will "turn into a snake," and will show you this long threadworm in a horse trough to prove it. I never knew a cautiously made experiment in that direction to succeed; nevertheless the fanciful error survives. The gordius, which does look like a hair from a gray mare's tail, is somewhat aquatic in its habits.

[CHAPTER VII]
DWELLERS BETWEEN TIDE MARKS

THE COLONIAL MOSS ANIMALS

Seaweeds and rocks at and below the limit of the ebbing tide are often covered with small bushy growths, or with lacelike incrustations that are alive. These are moss animals, representing the class Polyzoa of the phylum Molluscoida. They are minute, soft creatures that live in colonies formed by the repeated budding of the members, all connected by a fleshy base so that each contributes to the nourishment of all. "Each little animal occupies a separate stony or horny capsule, into which it may withdraw and even close the opening with a lid.... The mouth is surrounded by tentacles that in many species arise from a horseshoe-shaped or disklike base. These tentacles are always beset with hairlike bristles which by their movements serve to set up currents, and thus to drive minute organisms into the mouth."

A typical example of these polyzoans (or bryozoans) is Bugula turrita, so abundant wherever our northeastern coast is rocky that the rocks below tide level appear covered with its mossy tufts, which are often ten inches long and profusely branched. The main stems are orange-yellow, while the terminal branches are yellowish white. The delicate tracery so frequently seen on the fronds of kelp, and on shells and stones along both shores of the Atlantic indicate colonies, or their remains, of the lace coralline (Membranipora); and the dull red or pinkish crust so common on shells and stones in shaded tidepools represents successive colonies of the "red-crust" polyzoan (Escharella variabilis), layer crusting over layer. A similar history accounts for the curious nodules called "false coral" so common in moderately deep water in Long Island Sound. Similar polyzoans, which exist in great variety, both modern and fossil, contributed extensively to the formation of the older strata of sedimentary limestones.

ANCIENT LAMP SHELLS

Associated in structure with these minute colonists is the ancient race of brachiopods (Brachiopoda, "arm-footed") or lamp shells, although they much more nearly resemble bivalved mollusks, whence, by the way, comes the name of the phylum to which both belong—Molluscoida, which means "mollusklike."

The race of the brachiopods goes back to the beginning of the geologic record. A few living examples are still found in the ocean, some of which, as lingula, have changed so little that they can hardly be told from the most ancient fossils of their family. Certain species are dredged abundantly on both coasts of the Atlantic from water a few fathoms deep where the bottom is rocky. They look like small mussels at first sight, but on examination show a vast difference in structure. The bivalve shells, instead of growing on the right and left sides of the animal, as in bivalve mollusks, cover its back and front, and the head parts are at the gape of the valves. At the hinge end of the shell the lower valve overlaps (it is the shape of this lower shell, like that of an old Roman lamp, which suggests their common name, "lamp shells") and the hinder end of the body projects as a stalk, by which the animal fastens itself to the rock. "The mouth in the brachiopods is flanked by two curiously coiled and feathered arms which lie within the cavity between the shells, and are supported by skeletal rods attached to the upper shell. These serve as gills, and also to capture the minute creatures upon which the brachiopod feeds."

Owing to their great abundance, world-wide distribution, and remote antiquity, as well as their excellent state of preservation, brachiopods occupy a very conspicuous rank among extinct invertebrates, and furnish us besides with a large number of important index fossils. They are to be found in immense variety from the Cambrian to the present, most numerously in formations from Silurian to Permian times.

STARFISHES, SEA URCHINS, AND TREPANGS

We have now arrived at the point (phylum Echinodermata, "spiny-skinned") where a distinctly new type of interior structure appears in the possession by animals of a hollow space (cœlom) between the outer skin and the wall of the digestive tube which now becomes occupied by definite organs instead of by an almost uniform mesenchyme, as in the sponges and cœlenterates. These organs arise from an interior lining membrane called "mesoderm."

Henceforth, therefore, we shall deal with cœlomate animals, among which the echinoderms are lowest in rank. The simplest of them is the "sea lily" which lives rooted on the bottom in deep water, and sways about on a slender, jointed stalk, looking much like the flower after which it is named. It is of interest chiefly as a survivor of the tribe of crinoids that were so varied and numerous in early Paleozic times that massive Devonian limestones are composed largely of their remains; and the type has changed little through the ages. It consists typically of a cup, mounted on its stem like the calyx of a flower, and composed of circles of calcareous plates, definite in form and in relative position, that contain and protect a well-organized body. Surrounding the open mouth of the cup is a circle of long, jointed, much-branched tentacles that sweep the water, capture passing prey, and bring it into the mouth of the crinoid within the circling base of the arms.

If now you were to cut off its stalk, lay the crinoid on the sand, mouth down and arms outspread, beside a brittle star or a basket fish, which also have many-branched arms, it might be difficult to tell them apart, yet they represent different orders; and from this, by way of the naked serpent star, it is but a short transition to the starfish, where the arms are no longer tentaclelike, but are simply pointed extensions of a central body; this, in fact, is the case, for now they are no longer prehensile organs, but are supports, mainly serviceable in locomotion, and the stomach and ovaries are partly lodged in them. The main point just now, however, is the fact that here, and in the successive changes of form to be shown, the pattern of plates that form a strengthening mosaic in the skin of the central part of the body remains identical.

All starfishes are not as prettily symmetric as our familiar five-finger. Some are shorter in the arms, and much broader and thicker in the body; and if you will examine a collection of preserved specimens of the echinoderms you will see that you can trace gradation of form right around to the bun-shaped cake urchin, on whose top the five-pointed star is printed, and thence to the globular sea egg, which the French called "sea urchin," using one of their names for the hedgehog. Furthermore, the five sections of the shell of the urchin, which represent the five arms of the starfish folded forward and grown together into a spherical case, are to be traced again, outlined by appendages, in the elongated and leathery hide of the trepangs and sea cucumbers of the order Holothuria.

It is as an illustration of homology, that is, the resemblance between parts that have the same relation to the typical plan of structure, and as an example of how almost endless variations of form may arise within a single type, that the echinoderms are of most interest. Otherwise it may be said that they serve as food for fishes and some other creatures, including coastwise savages, and as curiosities in geological museums and in aquaria; and that starfishes are sadly destructive of cultivated oyster beds. We may therefore dismiss them, and devote a page or two to the worms.

EARTHWORMS AND BEACHWORMS

Although various parasitic creatures have been described as flat "worms," round "worms," and so forth, naturalists regard as true worms only those of higher organization classified in the phylum Annulata, or annelids, the distinctive characteristic of which is that its members have elongated bodies divided into ringlike sections. These represent a division of the internal parts into a series of structural segments or "matemeres," each supplied with its own set of organs, yet connected by blood vessels and nerves, and the whole traversed by tubular organs serviceable to the entire animal. The nervous system consists of a "brain" in the head, and a double, ventral nerve-cord with a ganglion in every segment, foreshadowing the nervous system in insects and other arthropods. The phylum embraces three classes: 1. Chætopoda—earthworms and marine annelids; 2. Gephyrea—marine worms, otherwise called sipunculoids; and 3. Hirudinidæ—leeches.

The earthworm or "angleworm" (that is, angler's worm, bait worm) of the "common garden variety," to use the phrase of old-fashioned encyclopedias, is a typical example of the first class, whose Latin name refers to the bristles (setæ) on the flattened lower surface of the body that serve the worm as "feet." A magnifying glass shows them in four double rows allowing eight to each of the rings into which the body is so plainly divided; their extremities are directed rearward, and by their means the worm pushes itself along, and is able to cling to and climb not only the walls of its burrow but vertical surfaces when not too smooth. Thus they are found frequently on roofs and in other elevated and surprising places, to which they have crawled in the night, when, as well as in warm, rainy weather, they are likely to wander a great deal. The long and greatly extensible and elastic body tapers almost equally at each end, but the head end is that which goes forward in crawling, and a lens will show a mouth on its lower surface, beneath a sort of thick lip. A long gullet leads into an expansion called the crop, and that into a large, tough-walled stomach, beyond which an intestine leads to the last segment. The thirty-third to thirty-seventh segments are swollen, forming the "belt" (clitellum), which denotes maturity, but seems to have no special functions. The senses are few and dull. No eyes exist, nor sense of hearing, but the skin is extremely sensitive to vibrations, and to bright light, as might be expected in a nocturnal animal. The sense of taste is discriminating. The eggs are extruded in such a way as to form a glutinous ring about the body, which, when complete, is slipped over the head of the worm, and left to hatch in warm soil under a stone.

Earthworms live underground in burrows that are sunk well below the frost line. In digging they work head downward, gnawing—although they have no hard jaws—and swallowing the earth that is not easily crowded aside and then throwing it out and perhaps heaping it up as "castings." The tunnel must be wide enough to let its occupant turn around in it, and it ends in a deep chamber in which one or more worms may pass the winter without freezing. These worms naturally seek a loose, damp soil, not only for ease of working, but because moisture is a necessity, as they breathe through their skin; hence they abound in meadows and cultivated soil, and are not found on high, dry plains. During the day they lie near the surface, often with the head just protruding. Here they are discovered by sharp-eyed birds and garter snakes, and sacrificed by thousands, notwithstanding the strength with which they hang on to their retreats by the tail. When it retires to the depths of its burrow this worm plugs the mouth of the tunnel with leaves which it draws always by the base, exhibiting considerable intelligence in manipulating the various shapes of leaves to that end.

The world-wide distribution of the earthworm is to some extent owing to man's agency. On our northwestern plains, for example, these worms originally were absent, but are now widely distributed and flourishing there, having been carried from the east, as eggs or small worms, in the soil packed about the roots of trees and shrubs transplanted to western orchards and gardens. This fact may have something to do with the recent westward spread of the robin, which, more than any other of our birds, is a hunter of them. Except where excessively numerous these worms do far more good than harm in a garden.

The naids (Naidæ) are small transparent worms that creep about on vegetation in fresh water, and, besides laying large eggs, they occasionally divide into two at a place in the body that appears arranged for this purpose, for it consists of a zone of very elementary tissue. "Gradually," as Minot records, "the tissue of this interpolated zone transforms itself into muscles, nerves, etc., and, growing meanwhile, it forms in front a new tailpiece to patch out the anterior half of the worm, and behind it forms a new head for the posterior half of the original body. The zone then breaks and there are now two worms." A relative, the lumbriculus, does the trick in a much more prosaic way, breaking in two first, and letting the separate halves acquire head or tail as best they may. This ability to reproduce lost parts is of much service in the life of the species and often of the individual, which may still live after some water tiger has bitten it in two—and these worms are at the base of the food supply of rivers and ponds, and would soon be exterminated were they not capable of rapid and profuse multiplication.

Worms of this class dwell in great numbers and variety in the sea and in salt-water meadows and beaches, and are often beautiful as well as interesting objects of study for the visitor at the shore. The sea mouse (Aphrodite), for instance, which is about three inches long and of oval shape, is covered with hairlike bristles that glisten with brilliant green, red, and yellow iridescence; it is to be looked for on the mud just below the low-tide line, and inhabits both coasts of the North Atlantic. The body of the common "clay worm," dug for bait at low tide, which is olive in general tone, gleams with pearly iridescence, while its innumerable feet bear gills that are green and salmon-red. Another (Lumbriconereis) is known as "opal worm" for good reason; and our sands abound in slender scarlet worms of the same genus named "red thread." All these worms bury themselves in the sand, or wander through it in search of prey, for they are carnivorous, and do not hesitate to kill and eat each other. Some are fairly sedentary, and protect themselves against fishes, crabs, mollusks, and bigger annelids that seek them, by forming tubes by means in some cases of a shelly secretion, but more usually by cementing bits of shell, stones, and grains of sand into an irregular tube lining the burrow; the slender, limy serpentine tubes often seen on stones or dead shells in tide pools, are, or were, the homes of such protected worms, most commonly of the "shell worm" (Serpula). "Often a number of these calcareous worm tubes are seen clustered together. When undisturbed the worm protrudes its beautiful feathered gills, which resemble a little passion flower projecting from the mouth of the tube. These gills are variously colored in different individuals, some being purplish brown, banded with white and yellow, while others are yellowish green, orange, or lemon-yellow. At the least disturbance, such as a shock or a shadow, the gills are instantly withdrawn into the stony tube, and the opening stopped by a horny disk." In the Gulf of Mexico extensive colonies of these worms often form, and as the early generations die others erect their tubes above them; as this goes on sand and shell fragments fill around and between the tubes, and after a long time the whole mass becomes a solid reddish, loose-lying rock, composed chiefly of serpula tubes, which in Florida is dragged up from the beach and used as building stone.

The third class (Hirudinidæ) of Annulata is that of the leeches, those ugly, but useful, worms of land and sea. In spite of their sluglike appearance the leeches are segmented worms, although the wrinkles on their gray, mottled skins do not indicate the position of the segments beneath. The mouth on the under side of the head is armed with jaws and sharp teeth that make three or more cuts through the skin, whence the blood is sucked; there is also a holding sucker near the tail. Their attacks cause little pain, and that fact has led physicians to put them into use when bleeding is required. The eggs of leeches are laid in moist earth in little packets, and hatch in five or six weeks. The growth to maturity is slow, and continues during a long life. Many species abound in ponds and stagnant waters. Asia has terrestrial leeches, swarming in moist vegetation; and in Ceylon the minute leeches are a terrible plague in certain regions. Many also are wholly marine. Some of the larger forms attack fishes directly, and quickly kill them by sucking their blood away; others are true parasites. On the other hand the leeches of our lakes are fed on by the whitefish and similar fishes. They are a great pest to our fresh-water turtles.

[CHAPTER VIII]
BUILDERS OF THE PEARLY SHELLS

The mollusks, or "shellfish" (phylum Mollusca) are a homogeneous group of soft-bodied, unsegmented, typically bilateral, elaborately organized animals, mainly aquatic and marine, whose origin—probably as a derivative from a wormlike stock—is lost in the mists of geologic prehistory. In most cases the mollusks secrete from a larval gland an external shell which serves as skeleton and defensive armor; are bisexual and produce eggs, or if monœcious are never self-fertilizing. They possess a heart, and blood circulation (usually colorless); breathe in the water by means of gills, or, in the air, by a primitive kind of lung; have a nervous system and senses in some cases of a high order; the organs are normally paired, and protected by a general covering integument called the "mantle"; and the creeping species move by a muscular, elastic, ventral organ styled the "foot," while the swimmers are provided with a variety of swimming organs. Mollusks vary in size from all but microscopic minuteness to a bivalve weighing 500 pounds or a squid half as big as a right whale. They occur in all seas at all depths, abound in fresh waters both swift and stagnant, and are scattered over the earth wherever vegetation flourishes.

The phylum Mollusca is divided into five classes, as follows, and it will be noticed that four of the names refer to the locomotive organ or "foot" (Greek pous, "foot"):

I. Pelecypoda, the Mussels—mollusks inclosed in a bivalve shell fastened by a muscular hinge, the adjacent part of the valves being generally more or less toothed; the foot is as a rule roughly comparable to the shape of an ax head.

II. Amphineura, the Chitons—flattened, bisymmetrical mollusks whose shell consists of eight crosswise, overlapping plates.

III. Gastropoda, Snails, whelks, etc.—mollusks that crawl on the flat undersurface of the body, or distensible foot.

IV. Scaphopoda, Tusk shells—mollusks that possess a long tubular shell open at both ends; with their small and elongated foot they are supposed to dig into the mud in which they live.

V. Cephalopoda, Cuttlefishes, and Octopods—mollusks with tentaclelike "arms" arranged about the mouth, and either an external or internal shell. These are the highest in rank.

THE OYSTER AND ITS RELATIVES

The lowest in rank of these classes is the Pelecypoda, containing the "bivalves"—mussels, clams, oysters, and the like, in which the shell is in two parts or valves hinged together over the "back" of the animal, and attached to it on each side by a powerful muscle, the "adductor," by the contraction of which the shell may be tightly shut. Within the shell the body is enveloped in a "mantle," or fleshy membrane falling like a cloak on each side; and from it is secreted the outer shell, which grows by additions to its ventral margin. These additions are in a general way annual, so that the concentric lines of growth on its exterior are an indication of the years of the mollusk's life, which is slow in growth, and long-lived. The interior of the shell is usually pearly, and marked with microscopic rugosities, which, by breaking up the light, as if by innumerable prisms, gives the iridescence so beautiful in the pearl oyster, the fresh-water unios and many others. These pearly layers are called "nacre."

Bivalves were formerly classified in conchology as Acephala, because they have no proper head, but at the posterior end are two openings of tubes, provided with cilia. In one, the cilia induce a constant current of water which after leaving the gills brings into the animal's stomach floating microscopic food, both plants and animals, including eggs and larvæ, where it is captured and assimilated while water is ejected through the other (dorsal) pipe. This food includes bacteria, and if the mollusk lives and feeds in water polluted by sewage, or otherwise containing germs of disease, it becomes dangerous as human food; hence oysters and clams exposed to such bad conditions ought never to be sent to market because of the disease germs remaining in them.

In bivalves such as the oysters, horse mussels, piddocks, and others that are sedentary, and often fixed in place, or that, like river mussels, scallops, etc., move about freely, the mouth tubes are short; but many bivalves, as the clams, pinnas, razor fish and so forth, bury themselves in the sand of the bottom, by means of the strong distensible foot protruding from the forward end of the shell. These are provided with a double-barreled tube, called the "siphon," which may be contracted within the protection of the closed shell, or may be stretched out several inches; the animal may thus sink its body deep in the sand while its siphon reaches to the surface and inhales food-bearing water. The little squirts of water often seen jetting out of the beach at low tide as one walks along it are from clams so buried, and which, alarmed by the vibration of one's footsteps, hastily eject the water and withdraw their siphons.

The old name for this class, Lamellibranchiata, referred to the gills, two of which, on each side, hang like curtains inside the mantle and between it and the saclike body containing the viscera; when the shell is open they are laved by the water, and extract from it, by some quality hardly understood, the oxygen necessary to regenerate the blood that flows through them; and, in addition, respiration is carried on through the skin.

The nervous system is very primitive, and the sense organs consist of an otocyst (a minute sac in which a hard particle floats in a liquid) in the foot, by which, it is believed, a sense of direction is had, and which also serves the purpose of an ear; an organ that tests the water; and in some, as the scallop, rudiments of eyes are situated on the margin of the mantle. Most pelecypods are of two sexes, but some, such as our American oysters, are hermaphrodite. Eggs in vast number, and a cloud of spermatozoa, are thrown out in midsummer, and a little of the latter succeeds in reaching and so fertilizing fortunate eggs, but almost all merely serve as food for the host of mollusks, worms, sea anemones and what not that subsist on such provender. The few fertilized larvæ drift about and happily escaping multiplied perils, presently settle to the bottom to attach themselves to some fixed object, or otherwise get a chance to grow big enough to defy ordinary enemies. Some interesting variations in this rather commonplace larval history occur, however, in certain families.

It will be possible to name only a few of the most useful or otherwise conspicuous bivalves, beginning with the oyster, concerning which an immense amount of detailed information is accessible to the reader in the reports of the United States Government (Tenth Census, and documents issued by the Fisheries authorities) and in those of States, like Connecticut, New York, and Maryland, where oyster culture is an extensive industry, said to be worth in the aggregate about $20,000,000. The oyster of the eastern American coast is to be found in the Gulf of St. Lawrence, but not in considerable numbers between there and western Maine, whence it is present southward to the Gulf of Mexico, except on the shifting sands of the outer beaches. It seeks protected waters and a rocky or weedy bottom furnishing objects to which it may, when young, attach itself, and later will not be torn adrift by storms, for where an oyster establishes itself in infancy it means to stay all its life. Hence the sheltered waters of Buzzards and Narragansett Bays, Long Island Sound, and the lagoons and inlets that lie behind the outer line of sandy beaches from Long Island to Florida are the sources of our supply—especially Chesapeake Bay.

A full-grown oyster will produce about 9,000,000 eggs, each being about one five-hundredth of an inch in diameter. When the little oyster (spat) is about one-eighth inch wide shells begin to form on its sides, and it settles to the bottom with its left side down, usually where other oysters are; and hence extensive colonies, or "reefs," of these mollusks form, and "rise on their dead selves" to a level where they may be reached by the oysterman's rake. Many years ago, however, it was discovered that large, marketable oysters were becoming very scarce. Oystermen therefore sought favorable places, and raking the natural beds transplanted their catch, little and big, to new ground, where they were left to mature. This crude method was next improved on by sowing thickly over the new ground, just before spawning time in midsummer, a great quantity of empty oyster and other shells. These were favorable to the catching of "spat," and would result in a new bed that in about four years would furnish salable oysters; and annual plantings produced, after a time, an annual crop. These are the essential facts of oyster culture everywhere, although methods differ somewhat in other parts of the world—in France, for example, fascines of twigs are spread over tidal flats to catch the spat, instead of shells.

Our eastern American oysters are undoubtedly the largest and finest for the table of the many species that exist all round the globe. Those of the Pacific coast of the United States are excellent, but small; and the same is true of the European species; nor is the use of oysters abroad so general and extensive as in the United States.

The pearl-bearing oysters are somewhat distant relatives of the edible oyster (Ostræa), the thorny oysters (Spondylus), the hammer shell, the windowglass shell (Placuna) and others. The pearl oyster of commerce is named Meleagrina margaritifera and is found in scattered localities within the tropics on both continents. The chief fisheries are in the Persian Gulf, around Ceylon, in Australia, among the Sulu Islands and on the west coast of Panama. The Pearl Islands, south of Panama, yielded to the early Spanish adventurers riches in gems that rivaled those their competitors obtained from gold mines; but now they are a field of small importance. In fact, the pearl fishery is carried on now far less in hope of a profitable collection of gems than for the profit in the shells, which have a nacreous interior of remarkable beauty—the mother-of-pearl—and the great advantage of offering this in almost flat surfaces, sometimes eight or nine inches broad, making it useful in the arts as well as in the more practical line of buttons, knife handles, etc. Sometimes the whole surface of a fine shell has been carved, cameowise, with cunning art and an exquisite effect.

[CHAPTER IX]
BUILDERS OF THE PEARLY SHELLS—Continued

MUSSELS, SCALLOPS AND CHITONS

The familiar marine mussels of the family Mytilidæ will some day become of great importance in this country as a food supply, as now they are useful in resisting encroachment by the sea on certain parts of the coast. They exist in vast numbers on both our coasts, and elsewhere in the world, in two genera, Mytilus and Modiolus, which differ a little in form, but not in habits. They have acquired the stationary habit, and in place of a "foot" of serviceable size have developed a gland that secretes an exceedingly tough, fibrous bunch of threads known as a "byssus," by means of which the animal may not only attach itself firmly to any sort of object, but may actually move about. The common species of Modiolus, the "horse mussel," lives in great numbers north of Cape Hatteras at and below the line of low water, and is much larger than the edible mussel just described. A smaller species of Modiolus is extremely numerous on the New England coast, and down to the Carolinas, forming dense tangled beds on muddy patches as well as among rocks, and serving to bind the mud and plants together and hold them from disintegration by stormy waves, in spite of the thin and brittle character of their shells. A southern species is bright yellow, with dark rays; and the common modiola of the Pacific coast is dark, glossy brown. Such mussels are eaten regularly in Europe, and come to us in a pickled condition as a luxury. There is no reason why we should neglect to add our own to our long list of sea foods.

The next useful mollusk to be considered is the scallop, one of the many species of the family Pectinidæ, of which we eat only the adductor muscle. The commercial species is Pecten irradians, the name referring to the (nineteen) ridges that radiate from the flattened hinges to the scalloped margin of the shell, which is prettily colored. This species is common in sandy, shallow places from Cape Cod to Florida, but the fishery is most productive about the eastern end of Long Island and in Narragansett Bay. Farther north is a very much larger species (P. islandicus) especially abundant on the Grand Banks, off Newfoundland, where it forms an important food of the cod and other fishes. It is well known to cooks, who use it in baking their fish confections en coquille. A large number of other species are distributed throughout the world, one (P. jacobæus), inhabiting the Mediterranean having the name "pilgrim shell" in allusion to the fact that in the days of medieval religious pilgrimages, those who had visited the shrine of Saint James at Santiago de Compostela, Spain, to pay homage on July 25, were accustomed to wear a scallop shell in their hats in token of the fact—this mollusk being connected with traditions of that saint.

Turning to the fresh-water mussels, or naids, as some books call them, one is staggered to learn that more than 1,500 species have been named, a large proportion of which belong to the United States, which is peculiarly hospitable to them because of our many rivers and lakes, together with the prevalence of limestone rocks, whose constant dissolution in water supplies the store of calcareous matter that these thick-shelled mollusks require. All belong to the family Unionidæ, in which two divisions are noted—one (Anodon) in which the mussel has a comparatively elongated thin shell with no "teeth" in the hinges; and the other (Unio) in which the shell is thick, various in shape from an oval to a triangle, and has prominent umbones, beneath which the valves (which are always alike) are hinged together by interlocking teeth embedded in a somewhat elastic gristle. The interior of all these unios is richly nacreous, and consequently pearls are produced in the same way as in the marine pearl-bearing shells; and some of the finest known gems have been derived from them, in this country and abroad, as well as innumerable specimens of moderate value. These mollusks like clear streams or lakes with a sandy bottom, and are not to be looked for in stagnant weedy waters. They keep an erect position, the nibs of the shell half buried in the sand, and move slowly about, plowing a path and dragging themselves along by means of the powerful foot, but keeping the short siphons at the other (or longer) end of the shell well above the mud.

We come next to our market clams. These are of two distinct kinds—"hard" and "soft," or quahog and long clam, as they are distinctively called. The quahog is a thick-shelled, roundish mollusk with a distinctly heart-shaped outline when looked at endwise. It dwells in fairly deep water, standing on its nibs half buried in the sand, like a wedge, and moving slowly about. Young ones become the "little necks" of our summer tables.

The soft clam belongs to a different race. Its elongated shell is thin and chalky, is loosely hinged, and gapes widely at both ends, and although it is used much as food, especially in chowders, it is by no means as good as the hard clam. Its principal value, indeed, is as bait in the cod fisheries, and for this purpose enormous quantities are gathered. It lives in, rather than on, muddy beaches, sometimes in crowds of thousands, its shell deeply buried, and its long siphons reaching up to suck in water and food when the tide covers the flat. When the tide is out, a tiny hole in the sand and a spurt of water show the clammer where to dig, and his spade quickly unearths the clam.

The second class, Amphineura, contains the chitons and their relatives. These chitons are flattened mollusks protected by an armature of eight crosswise plates, overlapping like shingles, which creep about the rocks close to shore, and when lifted curl up like sowbugs. The most interesting thing about the chitons is the fact that they are provided with excellent visual organs, "the whole dorsal surface of some forms being studded with eyes, of which not less than 8,000 occasionally exist on a single specimen." Many of them are complete, with cornea, lens, and a pigment layer within the iris.

SNAILS AS TYPES OF GASTROPODS

The gastropods (Gastropoda), including the snails and slugs, limpets, whelks, periwinkles, sea hares and the like, are Mollusca having the mantle completely enveloping the body, and the shell, when present, in a single piece, and usually in spiral form. There is a well-developed ventral foot, on which the animal creeps, and in front of it a distinct head bearing eyes and tentacles. These organs retain their normal bilaterality, but the body is, as a rule, inequilateral. The cause of this is the fact that on the animal's back is developed from the first a shell, which, with its contents, amounts to a relatively large weight, and it naturally falls over to one side. The mouth is armed with a flat, distensible, ribbon-like organ, studded with rows of chitinous teeth, that serves as a rasp and a boring instrument, and which is called an odontophore, or, in snails, a radula. Most gastropods are carnivorous.

The lowest in rank are the shell-less, or "naked" gastropods known as "sea slugs," "sea hares," and so forth. One Mediterranean species of Aplysia secretes a purple liquid utilized by the ancients as a dye, and this is still sought for in Portugal, where storms sometimes cast vast quantities of the mollusk on the beaches.

We come now to the great group of mollusks inhabiting fresh waters and dry land—the snails, whose group name is "pulmonates," that is, possessors of lungs, and breathing air. On the generally accepted theory that all these are descended from marine ancestors, and have gradually acquired the faculty of living on land, it would be natural to look for a series of mollusks that were amphibious, and, as it were, half-way fitted for a terrestrial existence, and such intermediates exist in all parts of the world. The little black Melampus, which covers the mud of tide flats on both the Atlantic and Pacific coasts in tens of thousands, and seems just as happy when the tide is out as when it is in, or when it is simply refreshed by the spray, is a good example. A near relative, Carychium, is still more emancipated from the sea.

First among these pulmonates are those common in ponds and still streams the world over, of the family Limneidæ, called limneids or pond snails. They are in various forms. Some are limpet-shaped (Ancylus), some are flatly coiled (Planorbis), but most of them have shells drawn out into a graceful spiral; in all cases the shell is not composed of lime, but of the thin, fragile, horny substance "chitin." The best known one is Limnea stagnalis, which sometimes reaches a length of two inches, and inhabits almost every quiet piece of water in North America, and in Europe and all Asia except India and China.

These water snails of our ponds and ditches are exclusively vegetable feeders, and must come to the surface at frequent intervals to breathe, letting out a bubble of vitiated air, and taking in a fresh supply. Should the pond dry up in summer the limneids burrow down into the mud, and remain in that heat trance called æstivation until the autumnal rains refill the basin and let them come forth. The small kinds called "physas," exceedingly common everywhere in this country and Europe, differ from Limnea in having the shell partly enveloped in the turned-up fringed edges of the mantle, and by being coiled from right to left instead of clockwise. This reversal occasionally occurs in individuals of all gastropods, which are then said to be "sinistral," as opposed to the normal "dextral" coiling; but in the physas it is the rule.

Next come the wholly terrestrial pulmonates—snails and slugs, distinguished from the pond snails, which have only one pair of tentacles at the bases of which the eyes are embedded in the skin, by having two pairs of "horns," one of which carries the eyes on their tips—good eyes, which may be quickly withdrawn out of harm's way by inversion of the tubular stalks. The thick, extensible foot is surmounted by a body coiled within the shell; and this foot secretes a viscid fluid that lubricates the creature's path, and often leaves a silvery trail.

Snails are mainly vegetarians. The mouth lies just under the front tentacles, and its upper lip is armed with a horny, crescentic "jaw." Within the mouth is the lingual ribbon, which may be brought up against the cutting edge of the jaw. This tongue is studded with rows of infinitesimal, flinty teeth, the radula of our big white-lipped snail, a quarter of an inch long, furnishing room for 11,000 of these denticles; and as all of them point backward the tongue easily seizes and draws into the mouth whatever the jaw nips off. Substantially the same sort of "tongue" is possessed by all the gastropods, but the arrangement and shape of the microscopic denticles is different in every species, and this is one of the "characters" used in classification. With it the carnivorous rasp away their food; and by bending it double and using it as a gimlet bandits like Nassa, the oyster pest, drill through other shells and devour the occupant. You may pick up on any seabeach scores of examples of the work of these borers. In Europe some kinds of slugs and snails do great damage in gardens, but we have little to complain of in this respect.

Largely dependent on moisture, the young snails that are hatched in midsummer at once seek retreats, and may be looked for under leaves, logs, and loose stones in the woods and pastures. Most American snails are solitary, and will be found lurking in the moss beside mountain brooklets—a favorite spot for the glassy vitrinas—hiding in the crevices of rocky banks and old walls, crawling at the edge of swampy pools, creeping in and out of the crannies of bark on aged trees, or clinging to the underside of succulent leaves. Some forms, very beautiful in their ornamentation when magnified, are so minute that they might be encircled by the letter o in this type, yet you will soon come to perceive them amid the grains of mud adhering to the undersurface of a soaked chip or rotten log.

For fresh-water species various resorts are to be searched. Go to the torrents with rocky bottoms for the paludinas and periwinkles (Melania); to quiet brooks for physas and coil shells (Planorbis); for limneas to the reeking swamps and weedy ponds. By pulling up the weeds gently, you may get small species that otherwise easily escape your dipper or net. In the Southern States and in the tropics certain forms are to be picked off bushes and mangrove trees like fruit, especially the round "apple snails" (Ampullaria) as big as your fist.

SEA SHELLS IN NATURE AND ART

Other familiar forms of gastropods are the limpets, keyhole and half-deck; the abalones, so much used in the making of ornaments; and the many small sorts of "periwinkles" studding the rocks and hiding among the seaweeds of every coast. Then there are the pyramidal top shells (Trochus), the bulging, wide-mouthed turbans (Turbo), and the open-whorled wentletraps (Scalaria) which years ago were so rare that collectors paid $100 or more for a good specimen. The two former kinds are on sale in all seaside shops, with the natural rough brown exterior ground away until they gleam outside in the prismatic glory of the nacre layers that lie underneath. A group of heavy shells of carnivorous tropical mollusks furnishes ornaments for the mantelshelf also. These include the knobby volutes, often richly colored in marbled patterns or in spiral rows of round spots; the olives, whose ovate shells are sometimes dark purple, sometimes beautifully marked, and always glossy, because enfolded during life inside flaps of the mantle that completely protect them; the miters, that take their name from their resemblance in shape to the headdress of a bishop, and show splendid decorations in tints of red and orange; and the strong, spiny murexes, a small Mediterranean species, which is the principal source from which the ancients derived their Tyrian purple dye—a coloring matter yielded by treatment of the blood of many species, including one of the commonest little mollusks (Purpura) on our own coast, which old-fashioned New Englanders yet utilize sometimes for making an indelible ink for marking clothing. To this family belong the "drills" that destroy thousands of dollars worth of oysters annually in Long Island Sound by boring through them. Near relatives are the whelks (Buccinum), extensively eaten in England; and two of the largest and commonest shells on our eastern sand beaches, known to northern fishermen as "winkles" and along the southern coast as "conchs." These (Fulgur and Sycotyphus) are big, pear-shaped creatures with chalky white shells that crawl about near shore, seizing and devouring anything they can overcome, and working havoc on planted oyster beds; they deposit their eggs in parchmentlike capsules shaped like gun wads and connected into a long chain that are often thrown up on the beach, where they are called sea necklaces.

Of great beauty in their rich variety of color and pattern are the tropical cone shells, of which a large number of species are known, some so rare as to bring great prices in the conchological market. Their bite is poisonous. Equally numerous in species are the charmingly decorated auger shells, some (Pleurotoma) spindle-shaped, others (Terebra) that would serve as models for a church spire. Near them is classified that white mollusk (Natica) whose globular shell is perhaps the commonest relic of the sea seen on our northern beaches, and sometimes is as large as a man's fist; to it belong the curious "sand saucers" to be found in August, which contain its eggs. These naticas are predatory, and burrowing their way through the loose sand come upon and devour other shellfish, boring a circular, nicely countersunk hole through their armor and feeding on its inmate; their depredations on the northern oyster beds are a serious matter.

Well known and always admired are the cowries, smooth, brightly colored shells, shaped like an olive with a gash down the length of one side. This long and narrow aperture is usually toothed, and it is only in the young that any indication of a typical spiral growth is discernible. The money cowrie of Africa is small and cream-white.

Lastly a word must be said about the largest of known gastropods, the big "conchs" or wing shells (Strombus), the helmet shells (Cassis), and the tuns (Dolium). They are West Indian. The species most commonly seen in the United States, forming a border for flower beds in seaside villages, is Strombus gigas, with a delicate orange-red or pink interior, from which are cut most of the shell cameos offered to art lovers. This shell, like the great spiral triton of the South Seas, is also converted into a horn much used in foggy weather by the spongers and small coasters of Floridian and West Indian waters. The helmet shell, a heavy, rounder and smoother mollusk than the Strombus, is also extensively used in cameo cutting, especially the African black helmet, in which a white outer layer covers an almost black underlayer on the broad lip. Dolium has a large, globose but thin shell, ornamented with revolving ribs.

The class Scaphopoda is composed of a single family (Dentalidæ) known as tusk shells, because the little shells, one to two inches long, are shaped like an elephant's tusk, open at both ends. The structure of the occupant is so singular, the animal lacking head, heart, gills, and some other ordinary features, that naturalists believe it is a hopeless degenerate. One of the species of the Pacific coast is famous as the shell strung as ornaments and serving practically as money among the northwestern Indians until very recent times, under the name "hiqua."

NAUTILUS, DEVILFISH, AND SQUID

We have now arrived at the last and highest division of the Mollusca—the Cephalopoda, the class of the nautilus, ammonite, and other fossil forms, and of the squid, cuttles, and octopuses of our modern seas. The cephalopods are very different in shape, activity, and in their higher organization and intelligence, from other mollusks, but their general anatomy is the same. The special characteristic, as indicated by the name, is the fact that the head is surrounded by tentaclelike extensions of the "foot," which is here fused in part with the head, and divided into the long "foot arms," which are the instruments by which these predatory creatures obtain their prey. The underpart of the foot forms a tube called the funnel (or siphon). Through the funnel the animal expels water from the mantle cavity, and thus propels itself through the water. When the siphon is in its normal position the animal swims backward; but it can be turned back over the edge of the mantle, giving a forward movement. In cephalopods the sexes are separate, the male being often much smaller than the female. The eggs are usually laid in gelatinous capsules, commonly known in New England as "sea grapes," and the development is direct, that is, without any free-swimming larval stage.

The class is divided into two subclasses: 1. Tetrabranchiata, cephalopods with four plumelike gills inside the mantle; and 2. Dibranchiata, with only two such gills. In the first subclass belong all those very ancient cephalopods called in a general way ammonites, goniatites, orthoceratites, etc., that are found in such great numbers and astonishing variety in the Paleozoic rocks, from the Ordovician age onward, although but few groups survived beyond the Carboniferous period, and only two families can be traced as high as the Tertiary deposits, one of which—that of the nautilus—survives to the present day as the final remnant of one of the conspicuous and interesting populations of the primitive ocean.

The pearly or chambered nautilus is one of several species inhabiting the East Indies and the coral region of the South Pacific seas, creeping along the bottom in deep water, most numerously at the depth of about 1,000 feet. Hence the animal is not often taken alive, although the smoothly coiled and handsome shells are cast on the beaches in great numbers; and little is known of its habits or embryology. It is a soft lumpish sort of creature, with a great number of short arms and tentacles around the mouth, none armed with suckers. It begins life as a mere globule covered by a minute hood of shell; but presently, growing too large for this hood, it enlarges it by additions to the rim, and then forms behind its body a partition (septum) across the shell, cutting off the part in which it was born. As growth advances, this enlarging and partitioning continues until the nautilus has attained its full size. Then, as before, it occupies only the outermost chamber, behind which the whole interior of the shell is divided by the septa into chambers, abandoned and empty, but filled with a gas that buoys it up in the water. Oriental artists are fond of grinding away the dull exterior of the shell and exposing the gleaming nacre underneath; and of carving in this mother-of-pearl picturesque designs, examples of which are often to be seen in curiosity shops. This is not only the last remnant of the great group of ancient nautiloids, but one of the smallest, for some of the Paleozoic coiled forms were as big as a washtub, and the straight ones were often six feet long.

The Dibranchiata, on the other hand, are comparatively modern, as their ancestry dates back only to the Trias, and our seas still harbor a long list of living representatives. This subclass has two divisions: 1. Octopoda—octopods, the eightarmed argonaut and other octopuses; and 2. Decapoda—decapods, the ten-armed cuttlefishes, or calamaries, and the squids.

The octopods have a saclike body with eight arms of about equal size, in some kinds thick and short, in others long and snaky. Every arm has along its underside a double row of round, muscular suckers without horny rims; and whatever is seized by one or more of these arms is drawn into the mouth at their base, where it is bitten by a beaklike jaw of sharp horn, and further devoured by means of a toothed tongue similar to the radula of gastropods. Nearly all are tropical, but some species exist in deep water considerably to the northward. Certain species are used as food in many parts of the world, and are considered a delicacy in Italy and other Mediterranean countries. The fishermen of Japan and the Philippines capture them by the simple process of lowering big earthen urns and leaving them on the bottom overnight; when they are hauled up in the morning many will contain entrapped devilfish, as sailors call them, which at once go to market.

A very singular octopod is the little argonaut, or "paper sailor." Its body is not larger than a walnut—that is the body of the female, for the male is only a tenth of that bigness. Its home is mainly in the tropics and in deep water, but in the summer spawning season it rises to the surface, and is occasionally met with far northward on the Gulf Stream, drifting, apparently, in a snug little boat. The two dorsal arms are expanded into broad, roundish membranes at their ends, and old stories said that they were used as sails—a supposition of much use to poets; but the "boat," shaped somewhat like the shell of the nautilus, is not a shell proper, but a membranous pouch secreted by the mantle in spawning time, and not vitally attached to the body, but held in place beneath it by the two broadened arms, and serving as a receptacle for eggs and a cradle for the embryos hatching from them.

Turning now to the Decapoda, we treat of things much nearer home and familiar on both sides of the continent, for these are the cuttlefish and squids, none of which have an external shell, but possess an interior brace to their muscles either of lime or of chitin. The cuttlefish proper, or calamaries, are those of the family Sepiadæ, which have an oval, flattened body bordered by a fin; and two of the ten arms are, in the female, in the form of long, slender tentacles. In addition to being edible and easy to get, as they stay near shore, their calcareous back brace is the "cuttlebone" fed to cage birds; and they furnish the substance from which the drawing ink called "sepia" is made—principally in Rome. This is a brownish black liquid that the animal jets out through its siphon when it thinks itself in danger in order to make an inky cloud in the water behind which, as a sort of smoke screen, it may run and hide. Other cephalopods use this means of escape.

The squids, however, are all elongated in shape, and have finlike expansions of the mantle only on the tail. Two of their arms are long and slender, and are broadened at the tips, and studded with suckers. These suckers in some squids are strengthened by a horny rim, or by recurved hooks, or by both. The eyes are large, perfectly formed, and as serviceable as those of the fishes on which they prey. These, and some other animals, including small ones of their own kind, they capture by darting backward, swinging quickly to one side and seizing the victim in their sucker-bearing arms. They themselves are devoured by whales, seals, and many kinds of fishes; and enormous quantities of squids of various species are annually collected by fishermen for use as bait in the Newfoundland fisheries. In place of the calcareous cuttlebone of the sepia the squids have their bodies stiffened by an internal strip of chitinous substance called the "pen."

Squids are of all sizes from an inch to twelve feet in length; then there is a surprising jump to the giants (Architeuthis) of the North Atlantic, which, when the tentacles are stretched out in front, may measure seventy-five feet from tip to tail. These are little different in structure or habits from their smaller brethren that exist in so many species near all coasts and throughout the midseas right around the globe; but their huge size makes them fit antagonists of the sperm whale, which hunts them, and whose hide often bears a record, left by their powerful suckers, to show how hardly some big squid struggled for life. These monsters are the greatest invertebrates known in present or past time; and it is probable that the long wriggling arms of one and another, glimpsed at the surface, may account for some of the sea serpent stories brought home by apparently perfectly honest sailors, especially those which in many cases recount that the supposed "serpent" was in conflict with a whale. Carcasses of these gigantic squids are occasionally cast on the shores of Labrador and Greenland.

[CHAPTER X]
ANIMALS WITH JOINTED FRAMES

The phylum Arthropoda embraces an immense assemblage of small animals, inhabiting salt and fresh waters, the land, and the air above it. The typical members of this group have a body divided into segments, jointed limbs, some of which are modified into jaws, and a more or less firm external skeleton. The general organization is complex, with the nervous system and senses well developed, in some divisions showing powers of perception and brainwork of a very high order. The chief divisions, or classes, of the Arthropoda are given below in the order of rank, from those simplest in organization to the most complex. Members of the first three classes breathe by gills, and are termed Branchiata, the remainder are air breathers or Tracheata.

Crustacea—Crabs, lobsters, shrimps, barnacles, beach fleas.

Trilobita—Trilobites; eurypterids (fossil only).

Xiphosura—Horseshoe crabs.

Onychophora—Peripatus.

Myriapoda—Centipedes; millipedes.

Arachnoidea—Spiders, mites, ticks, scorpions.

Insecta—Insects.

As several of these classes contain many subdivisions, and thousands or even tens of thousands of species, all that is possible is to give the reader such an account of each important group, as will enable him to assign to their proper place such arthropods as he may encounter in his rambles, or in his reading, and to learn something of the manner of life in the various groups.

CRABS AND THEIR SMALL RELATIVES

"Everyone," says Dr. Calman, "has some acquaintance with the animals that are grouped by naturalists under the name Crustacea. The edible crabs, lobsters, prawns, and shrimps are at least superficially familiar, either as brought to the table, or as displayed in the fishmonger's.... Many, however, will be surprised that the barnacles coating the rocks on the seashore, the sand hoppers of the beach, and the wood lice of our gardens, are members of the same class. Still less is it suspected that the living species of the group number many thousands, presenting strange diversities of structure and habit, and playing an important part in the general economy of nature."

The great majority of crustaceans are aquatic animals, breathing by gills or by the general surface of the body, having two pairs of "feelers," or antennæ, on the front part of the head, and at least three pairs of jaws. Most crustaceans are hatched from eggs, usually in a form very different from their parents; and they reach the adult state only after passing through a series of transformations quite as remarkable as those that a caterpillar undergoes in becoming a butterfly. All crustaceans, except a few much modified land forms, breathe by means of feathery or platelike gills which are always an appendage of the legs, where they appear as one or more lobes. Colorless blood propelled by the heart wanders into spaces in these lobes, and there lies separated from the water by a mere film of tissue, through which oxygen is absorbed from the water. Most crustaceans are covered, at least in part, by some sort of shelly coat composed of a combination of the horny substance "chitin" with lime, which reaches its highest state in the big lobsters and crabs. This not only protects and gives support to the internal organs, but also to the muscles by which the animal moves. In other words it plays the part of a skeleton. As it does not increase in size after it is once formed, and cannot stretch much, the crab must cast its shell at intervals as it grows. The new covering, which had been formed underneath the old, before molting, is at first quite soft, and the animal rapidly increases in size owing to the absorption of water. The shell then gradually hardens by the deposition of lime salt.

The reader who may not hitherto have understood the difference between "hard" and "soft-shelled" crabs is now instructed; and it is observable that the figurative expression "a hard-shell," when applied to a man, signifies that he must undergo a complete change before his ideas will be enlarged.

The simplest of the crustaceans are those small creatures of the subclass Branchiopoda (gill-footed) that swarm in our waters, both salt and fresh. Lakes, ponds and ditches abound in a variety of minute or even microscopic species that, in gathering food from equally small bits of dead organic matter, as well as from living plants and animalcules, perform an important service as scavengers—a service, in fact, performed by all crustaceans in a greater degree than by any other single group of animals. They also furnish the basis of food for the whole body of aquatic life, since it is upon these minute crustaceans that fish fry, tadpoles, insect larvæ, caddis flies, and so on, must mainly depend. One of them is Daphnia, familiar to keepers of aquariums. Another is Cyclops, a favorite with microscopists and abundant in stagnant ponds, which is a member of the group called copepods that form an important part of the oceanic plankton, where they are the chief consumers of the minute algæ; but they also occur at all depths. In arctic waters the copepods are so abundant that they form the principal part of the food of certain fishes and of the whalebone whales. These, and their minute relatives, the ostracods, produce a large part of the phosphorescence of the sea, and some of them exhibit bright colors.

All these are free swimmers, but nearly related to them are the barnacles (Cirripedia) whose larvæ float about for a time near shore, and then settle down and attach themselves by their hinder parts to a rock or some other support, and begin to secrete an armature of limy overlapping plates that forms a strong cup in which they sit, often in a crowd that whitens a big rock. When the tide is low these sessile "acorn shells" are tightly closed, but when the water returns, bringing its load of invisible food, the animal stands up, as it were, and thrusting out its feathery legs sweeps the water to capture a meal—a beautiful sight to watch. The relation of the plates in the barnacle's cup to those in the coat of the higher Crustacea is more easily seen in the more pelagic "goose barnacle," whose hinder part is extended into a tough, flexible stalk, while the fore part is covered by plates. This kind is fond of attaching itself to floating timber, to ships' bottoms, or even to the surface of whales, and thus floats or is carried all over the watery globe. To it belongs the ridiculous myth of the barnacle geese.

Great numbers of crustaceans of more advanced types live in the open sea, and at all depths; and many of them assume extraordinary shapes. The space between tide marks, and the mud of salt marshes and tidal creeks abound in a wide variety of species, some of which are familiar to everyone who lives at or visits the seashore. Thus the sand and rows of drifted seaweed on all our eastern beaches are likely to harbor flocks of amphipods, well called "sea fleas" or "sand hoppers," which sometimes jump away before you in hundreds as you walk along.

Here, too, are to be found the pretty, burrowing "mole crabs," or "ivory crabs," so called from their shining white jackets; and a host of other species with strange forms and habits haunt the margins of tropical and Oriental seas. All these are bandits, preying on whatever they can catch, and between times guarding themselves from capture by fishes, bigger crabs, and other enemies, by lying in mud burrows, to the bottom of which they are quick to retreat. The big arm of the fiddler crab, held across its face, closes its burrow like a door. One sort, the hermit crab, has all its hinder parts naked, and so backs into an empty snail shell, curling its taillike soft abdomen around the central column of the shell and so dragging it about with it, with its armored head and thorax sticking out of the mouth of the shell. As it grows it becomes too large for its first shell, and from time to time must leave it and find a larger tenement in which to ensconce itself—a perilous transfer. Let me quote some notes I made on a New England shore to give a picture of crustacean life there in summer.

"The lady crabs were plentiful, always alert, and inclined to be pugnacious at our intrusion. The first one I met instantly rose upright at the surface of the water, and when I made an advance it sprang half way out of the water and cracked its pincer claws together as if supposing it would reach, or at any rate frighten me. Perhaps it was my shadow it clutched at so viciously. If so, the crab probably concluded its huge antagonist to be an intangible ghost upon which the most powerful claws could have no effect, for an instant later it backed down—literally and swiftly—to the bottom, and in a twinkling had wriggled tailwise into the mud and out of sight. When with my shovel I routed madame out of that retreat, she indignantly scuttled off too briskly to be followed, and will have great tales to tell of her adventure.

"The stone and fiddler crabs were as common and comical as usual; and I made the acquaintance of a new one called Gebia, which was a small, semi-transparent, bluish white, washed-out, bloodless specimen, shaped somewhat like a crawfish and carrying bunches of roe beneath its abdomen. It looked like a miniature lobster made of glass and filled with milk. Then in the eelgrass there was a funny isopod, called Caprella. It was half an inch or so long, and clung by its hinder feet to the grass, waving its body up and down in search of minute prey. Other isopods and amphipods were exposed by turning over stones or digging in the sand at the edge of the water—small, pale, shapeless crustacea, which are flattened laterally so that they must lie on their sides, and when uncovered will kick about with feet and tail in laughable anxiety to get under something. Under the stones we found the tubes made by a certain species; and when we captured the active little architect and put him in a bucket of clean water, he instantly began to gather grains of sand and stone and to join them together Into a shield under which he might hide. We found that these grains were joined together by spiderlike threads, which the amphipod spins from two pairs of small legs under the middle of his body, secreting a fluid that hardens in the water. Another (Hippa) about the size and shape of a robin's egg, but with a thin shell of mother-of-pearl (so to speak), gave us great amusement by its extraordinary celerity in burrowing, so that we could hardly seize it before it had squirmed down out of reach into the wet sand."

The edible crabs (Cancer) live in the shallow region just below ebb tide, for they cannot endure exposure to air as well as other species, and live by scavenging. The lobsters are inhabitants of still deeper water, especially where it is somewhat rocky, and devour more carrion than living fish. That miniature of the lobster, the fresh-water crawfish, which is also edible, dwells in deep burrows in wet lands—burrows that are really wells half filled with water. Various species of these and other edible forms of Crustacea are found all over the world.

MILLIPEDES AND CENTIPEDES

The myriapods (class Myriapoda) are those unpleasant creatures more commonly known as centipedes, millipedes, or thousand-legged worms. They have a wormlike form, with the body divided into segments, a distinct head with antennæ, jaws and several single eyes, and a varying number of air tubes, or tracheæ; two sexes exist, and eggs are laid in the ground within cases formed by the mother of pellets of mud. They vary in size from an almost invisible minuteness to a length in some tropical species of six or more inches. The centipedes (Chilopoda) are those flattened forms so often seen in and about rotting wood and vegetation or in moist ground, their bodies looking like a chain of plates joined together by flexible skin, each section having a single pair of legs, usually very short, but in one sort (Cermatia) each leg is longer than the body, and the hinder pair twice as long, matched by two very long feelers. Most of them are predacious, feeding on anything they can catch, and their strong jaws exude poison. The larger ones may inflict a very painful bite if incautiously handled.

Another group, the Diploda, are known as galley worms, or millipedes, and have two pairs of bristlelike legs on each segment. Here the body is as round as that of an earthworm, and is incased in a hard, chitinous shell, usually red-brown in color; and when disturbed they coil up and emit an acrid, unpleasant odor as a defense.

WEAVERS OF SILKEN TRAPS

The class Arachnida, which contains the scorpions, spiders, mites and their allies, connects the Crustacea with the Insects; and some naturalists include within it the eurypterids and king crabs, classified in this book with the Crustacea. All live on land and breathe air except a small group of allies (Pycnogonida) which are marine, and may be found on the rocks, and clinging to wharf piles, etc., on our coasts as well as elsewhere; they appear to be all legs, and are known to New England fishermen as "no-body crabs." The class includes seven orders, the lowest in rank of which is that of the scorpions (Scorpionida).

Scorpions are inhabitants of warm countries, and some tropical American species are six inches in length, but those of our Southern States are smaller. They have slender bodies consisting of a cephalothorax and a long abdomen ending in a sharp sting through which two poison glands inject poison into the wound made by it, the effect of which may be very severe on a man, and is fatal to the insects and other small creatures on which scorpions prey; this "tail" with the sting is usually carried curled up over the back. The body is protected by chitinous plates above and below. The legs are four. From the head spring two great, crablike, pincer claws. When these seize an insect they hand it back to two small but powerful appendages at their base which act as jaws. Between them is a small mouth. Scorpions are nocturnal in habit, hiding by day in crevices, and wandering about at night; thus they are likely to seek such dark retreats toward morning as a person's boots; and in hot, dry regions travelers must be cautious about examining their clothing and baggage to avoid being stung. The scorpions retain their eggs until hatched. The young when born differ little except in size from their parents, and are cared for with much solicitude by the mother, who carries them around with her for some time, hanging by their pincers to her body. The race is ancient, fossil remains occurring as early, at least, as the Carboniferous age.

The second order, Pseudoscorpionida, includes the "book scorpions," a series of minute, stingless, scorpion-shaped creatures found in moss, under the bark of trees, or more often on flies. A third order, Pedipalpida, is that of the scorpion spiders, or "whip scorpions" of the tropics; the fourth, Solpugida, contains certain ugly creatures intermediate between scorpions and spiders; and the fifth order, Phalangida, is that of the small-bodied, vastly long-legged things called "harvestmen" in England and daddy longlegs by us, which run about in the summer heat, and feed on minute insects. They abound in all the warmer parts of the world, and in great variety, South America showing some very bizarre forms. This brings us to the sixth order, Araneida—the spiders.

THE SPIDERS AND THEIR WEBS

Spiders are usually thought and spoken of as "insects" by the layman. Many persons call almost every creature an insect that is small and supposed to be useless, or suspected of harmfulness. But spiders are different from insects properly so called in many important particulars of structure and habits. Spiders have four pairs of legs, while insects have six legs. The spherical abdomen, which is cut off from the head by a deep constriction, shows no segmentation, and on its floor are large glands (the arachnidium) producing the silk which is exuded from three pairs of tubes with sievelike openings, at the end of the abdomen, called the spinnerets. Their nervous system is highly developed, and they show much intelligence. Spiders are of two sexes, but the male is usually much smaller than his mate.

When egg-laying time comes the female forms a little silken bed attached to grass, or underneath a stone, or stuck to some object, or placed in a burrow, or hung like a hammock by long guy lines, and deposits in it eggs like drops of jelly. One sort places this under water, forming a nest like an inverted cup and filling it with bubbles of air, and spending much of its time in this real diving bell. A common garden spider (Lycosa) forms globular cocoons, and drags them around attached to the spinnerets, regardless of jars and bumps. In a large section of the tribe this is all the use that is made of the silk, which differs from that of insects (caterpillars) in being made up of a great number of finer threads laid together while soft enough to unite into one.

It is a common habit with spiders to draw out a thread behind them as they walk, and in this way they make the great quantities of threads that sometimes cover a field of grass. This is the gossamer often so annoying to us in late summer, but a thing of beauty when glistening with dew.

The gossamer of autumn, however, is made by the very small spiders of the genus Erigones, which hide in the herbage, but in the fine weather that comes after the first frosts climb to the tops of posts, fences and tall weeds, in company with the young of larger kinds, and "turning their spinnerets upward allow threads to be drawn out by ascending currents of air, until sometimes the spiders are lifted off their feet and carried long distances." These are the "ballooning spiders" of which one hears. In this way the whole country is overspread with lines and tangles of trailing silken threads that whiten our clothes and stick to our faces.

Three or four hundred species of spiders might be obtained in almost any locality in this country by diligent search, and thousands of foreign species are known; hence only a few conspicuous examples may be mentioned here. The tribe may be divided according to habits into two groups of families: 1. The hunting spiders, which run on the ground or on plants, catching insects by chase or by strategy; and 2. The cobweb spiders, which make webs to catch insects, and live all the time in the web or in a nest near it.

In the former group are the Drassidæ, a family of small, varicolored spiders that run about on the ground or in bushes, one large genus of which (Clubiana) includes pale, or purely white species; their cocoons are baglike or tubular. The most conspicuous genus is Misumena, in which the species are white or brightly colored, and which spend their days among flowers, waiting in rigid attitude for an insect to alight near them on which they may pounce. Spiders can see well for four or five inches, but not much beyond that. The Attidæ are small, hairy, or scaly jumping spiders, often brightly colored, that are found in open places and on the tops of low plants, whence they leap on their prey, or make long jumps to escape danger. To the next family, Lycosidæ, belong the large spiders most often seen in fields and pastures. They are fond of dry, sandy places, where the females live in silk-lined holes. These lycosids are long-legged, rapid runners, and capture their game by running it down. To this family belongs the famous tarantula of southern Europe, fabled to produce a madness (tarantism) in a person bitten that could be cured only by dancing to music of a certain lively measure called "tarantella." (The so-called "tarantula" of our southwestern desert region, is, however, another species.) A common northern spider (Lycosa carolinensis) is its equal in size, (the longest legs covering a spread of three inches), and in color, black with gray legs. Still larger is another North American lycosid (Dolomedes tenebrosus), gray with spiny legs ringed with dark and light gray, which spreads four inches.

These big ugly creatures, and the bites of spiders generally, are regarded with unnecessary dread by most persons. The jaws (mandibles) are close together at the front of the head. They are two-jointed, the basal joint stout, and the end joint or claw slender and sharp-pointed. The claw has near its point a small hole, which is the outlet of the poison gland. "The poison kills or disables the insects which are captured by the spider. Its effect on the human skin varies in different persons. Sometimes it has no effect at all; oftener it causes some soreness and itching ... and cases have been known in which it caused serious inflammation which lasted a long time. Spiders seldom bite and only in self-defense, the bites so commonly charged to them being often the work of other animals."

In the family Agalenidæ we meet with the first of the web makers. These are spiders of moderate size, characterized by a big head marked off from the thorax by converging grooves. Their natural home is in the grass, where their flat, closely woven sheets of silk, almost invisible by reason of their transparency, but brought into plain view when coated with dew or dust, are spread everywhere. They also are fond of getting into cellars and old buildings, and constructing webs across corners, bracketwise. Somewhere the web sinks like a narrow funnel into a short tube in which the owner hides, watching hungrily until a fly alights on his silken platform.

"The Therididæ," says Emerton, "are the builders of the loose and apparently irregular webs in the upper corners of rooms, in fences and among rocks, and between the leaves and branches of low trees and bushes. They are generally small, soft and light-colored spiders, with the abdomen large and round and the legs slender and usually without spines.... Most of the Therididæ live always in their webs, hanging by their feet, back downward. The webs have in some part a more closely woven space under which the spider stands." These spiders are quick to avail themselves of any chance to spin their shapeless meshes of almost invisible silk, which few regard as real "webs," in closets, cellars, and all over the house or barn. Many of them are adorned with gay colors or striking patterns, and some are much feared, especially Latrodectus mactans, about half an inch long, which is black with scarlet spots. It is common from Canada to Chile, and everywhere is considered fatally poisonous—why, it is difficult to say.

Last of our list, and highest in rank, are the Epeiridæ, the "orb weavers," as they are often called, who make those regular spiral nets which are in our mind's eye when we think of cobwebs. Most of the moderately large and handsome house and garden spiders are of this family, and everyone can easily examine their work, although it is less easy to watch them at it, as the webs are built and repaired at night. Among the obscurer and foreign species the abdomen often shows humps, points and long forward-reaching horns that make them exceedingly grotesque, and doubtless difficult to handle by birds and other creatures that seize them as food.

One of the round webs of the Epeiridæ consists of several radiating lines, varying in different species from a dozen to seventy, crossed by two spirals—an inner spiral that begins in the center and winds outward, and an outer spiral that begins at the edge of the web and winds inward. The inner spiral is made of smooth thread, like that of the rays, to which dust will not cling; the outer spiral is made of more elastic thread which, when fresh, is covered with fine drops of sticky liquid.

"In beginning a web, after the radiating threads are finished, the spider fastens them more firmly at the center and corrects the distances between them by [inserting] several short, irregular threads, and then begins the inner spiral, with the turns at first close together and then widening
... until they are as far apart as the spider can
reach with the spinnerets [resting] on one and the front feet on the next, and so goes on nearly to the outside of the web, where it stops abruptly. The spider usually rests a moment, and then begins, sometimes at another part of the web, the outer sticky spiral.... As soon as the inner spiral is found in the way a part of it is cut out, and by the time the outer spiral is finished the inner is reduced to the small and close portion near the center.... The whole making of the web seems to be done entirely by feeling, and is done as well in the dark as in daylight. When the spider is active and the food supply good, a fresh web is made every day, the old one being torn down and thrown away."

As a rule these orb weavers do not stay in the web in the daytime, but hide away in their nests made in some near-by but concealed place; and their egg cocoons are hidden in all sorts of places.

All of the spiders that have been considered so far belong to the division of the class that has but a single pair of lungs. A second division has been made for those having two pairs of lungs, composed of a single family, the Mygalidæ, consisting of the so-called "bird-catching" spiders and the trapdoor spiders. The great mygale of Guiana has a body sometimes two inches long, and its legs will span eight or nine inches of space. It is hairy all over, intensely black, and a terror to all small creatures, even catching small birds, according to tradition; but proof of this is wanting.

The trapdoor spiders are those of the genera Cteniza and Atypus which dig and inhabit vertical holes in the soil, lined with silk and closed at the top by a hinged stopper or "trapdoor." Several species occur in southern Europe, one of which has a second door hanging by a silken hinge half way down the shaft; and in case of trouble the spider goes below it and pushes it above its head, so that the intruder is deceived into thinking it has opened an empty nest. Cteniza californica is the common species of our Southwest. The cover of the hole is made of dirt fastened together with threads, and is lined, like the tube, with silk, and fastened by a thick hinge of silk. The spider holds the door shut from inside. These underground homes are safe retreats for the spiders during the day, and nesting places in which their eggs are deposited and young reared; at night the spiders go forth in search of prey.

MITES AND TICKS

Mites and ticks are classified with the spiders as degenerate relatives of arachnoid stock. Ticks are large enough to be seen without a magnifying glass, and some become half an inch long. Ticks are wholly parasitic. The female lays several thousand eggs at one time on the ground or just beneath the surface. "The young 'seed ticks' that hatch from these in a few days soon crawl up on some near-by blade of grass, or on a bush or shrub, and wait quietly until some animal comes along. If the animal comes close enough they leave the grass or other support and cling to their new-found host." These parasites are the agents of the spread of several infectious diseases of cattle, the worst of which is the destructive Texas fever, and of mankind, as spotted fever and other ills resulting from the presence of blood parasites.

[CHAPTER XI]
FROM BUTTERFLIES TO BEETLES AND BEES

The generally accepted classification of the insects divides them into more than twenty orders, and these into hundreds of families whose species, already catalogued, are three times as numerous as all other known animals together. "There are, for example," as Lutz remarks, "15,000 species of insects to be found within fifty miles of New York City; more than 2,000 of these are either moths or butterflies."

Insects as a class are characterized primarily by the division of the body, when adult, into three clearly defined regions—the head, the thorax or fore body, and the abdomen or hind body. All insects have three pairs of legs, distinguishing them from the eight-legged spiders, and from the many-footed myriapods and other arthropods, and most of them have one or two pairs of wings, borne like the legs on the thorax, the abdomen never bearing either. The head consists of four segments, but in most cases the first three are consolidated into the hindmost, and are represented only by the appendages they bear. The foremost of these are the mouth organs, of which there are three pairs: the most anterior are the mandibles, next the maxillæ, and then the labium, the two latter bearing articulated prolongations known respectively as maxillar and labial palpi. The mouth has an upper lip (labrum) and contains a tongue. These mouth parts are variously modified, and by these modifications insects may be classified in two groups: "First, those in which the jaws and maxillæ are free, adapted for biting, as in the locust or grasshopper; and second, those in which the jaws and maxillæ are more or less modified to suck up or lap up liquid food, as in the butterfly, bee, and bug." It is in this latter group that we find those having those interesting relations with plants that result in cross-fertilization of flowers.

From the forehead spring a pair of antennæ, which are not only "feelers," but the bearers of other senses. They are jointed, and exceedingly various in form and service. Some are mere stubs, others long and slender as a whiplash, or they may be thickened at the end, as commonly in butterflies, or bear rows of hairs on each side, giving them in some cases a beautiful plumelike appearance. With their antennæ insects inspect by touch whatever they come in contact with, and test the shape of what they may be constructing, such as cells for their eggs. They recognize one another, and apparently exchange communications, or become aware of a stranger, and the ants induce their captive aphids to let down the honeydew by stroking them with their antennæ; but in many of these cases, if not all, additional information is derived through the antennæ by reason of the senses of hearing and of smell which many of them certainly possess. Ears, or organs sensitive to vibrations, and delicate hairs and other processes connected with nerves responding to touch are found in various other parts of insects' bodies, but the feelers are preeminently the seat of the sense of smell.

The eyes of insects are of two kinds, simple and compound. The simple eyes are small and practically useless single ones (ocelli) situated in a triangle of three on the top of the head. The compound eyes are on the side of the head, and are covered by a transparent layer of the chitinous skin (cornea), divided by delicate lines into square areas (facets). Beneath each facet of the cornea is an "ommatidium," optically separated from its neighbors by black pigment, and consisting of an outer segment or "vitreous body" and an inner segment or "retinula" formed of sensory cells. In some such eyes the ommatidia are few, but in others extremely numerous, so that the eyes cover a large space; some hawk moths are said to have 27,000 facets. The nature of the picture conveyed to the mind by such an eye has aroused much discussion. Photographs taken through the eye of a dragon fly show that, though the eye is compounded of many lenses and sensitive areas (retinulæ) corresponding to them, yet the whole eye throws one image on the retina. However complex such an eye may be, it is devoid of any focusing arrangement and can only receive a clear image when the retina and the object are separated by the focal length of the lenses. Hence the need for active movement on the part of creatures having them.

The head is connected with the thorax by a neck often protected by the overlapping front of the "tergum," or chitinous plate that covers the thorax. The thorax consists of three segments, named from the front backward "prothorax," "mesothorax," and "metathorax." These and a few other technical terms are in such constant use in describing insects that it is important to know them. The under (ventral) surface of the thorax is protected by another plate named "sternum." The armor is not continuous all around the body as in the crustaceans, but that on the upper surface is connected with the sternum by a seam of soft skin along the sides of the body.

Each segment of the thorax bears a pair of legs, each of which consists of a stout, flattened "coxa," nearest the body; a small second part, the "trochanter"; a third, the "femur"; a fourth, the "tibia"; and finally the "tarsus," or foot, terminating in a pair of claws, bristly on their under surface to give adhesive power. It is by means of these stiff hairs, and not by any suction or stickiness, that flies are able to walk on the ceiling and on vertical surfaces.

The wings of such insects as fly arise from the tergum of the thorax, and are in two pairs except in the flies, where there is but one, the hinder pair being represented by two little protuberances called "halteres." Usually the wings are strengthened by rods called "veins," and the patterns of venation vary in different groups, and form one of the means of classification.

The abdomen consists normally of ten segments, and contains most of the digestive and all of the reproductive organs, above which runs the main blood vessel, and below it the highly organized nervous system, the chief ganglion of which, in the head, is termed "brain."

The breathing of insects, although rhythmical in its inhalation and alternate exhalation, is not to the same purpose as ours. Respiration goes on by means of a system of branching tubes (tracheæ) that ramify throughout the body, and to which air is admitted through nine or more openings in the side of the body guarded by valves called "spiracles." The buzzing of flies, "singing" of mosquitoes, and the like, are sounds made in these spiracles, not by their rapid wings. At intervals the tracheæ are enormously enlarged to form air sacs. These no doubt, lighten the body, but they probably serve also to provide a reservoir of air from which the fine branches are filled by diffusion, and into which the carbon dioxide is discharged. The circulation of oxygen in adult insects, however, is never by means of the blood, but simply by absorption by the tissues into which the excessively attenuated tracheal tubes penetrate.

Insects are bisexual, and male and female are always separate individuals. Except in a few abnormal cases among the most lowly, eggs are produced and deposited in some favorable place for hatching.