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BEGINNERS’ ZOOLOGY

BY

WALTER M. COLEMAN

AUTHORIZED BY THE MINISTER OF EDUCATION FOR ONTARIO

TORONTO

THE MACMILLAN CO. OF CANADA, LIMITED

1921

Copyright, Canada, 1921

BY THE MACMILLAN COMPANY

OF CANADA, LIMITED

CONTENTS

BEGINNERS’ ZOOLOGY

CHAPTER I
THE PRINCIPLES OF BIOLOGY

Biology (Greek, bios, life; logos, discourse) means the science of life. It treats of animals and plants. That branch of biology which treats of animals is called zoology (Gr. zoon, animal; logos, discourse). The biological science of botany (Gr. botane, plant or herb) treats of plants.

Living things are distinguished from the not living by a series of processes, or changes (feeding, growth, development, multiplication, etc.), which together constitute what is called life. These processes are called functions. Both plants and animals have certain parts called organs which have each a definite work, or function; hence animals and plants are said to be organized. For example, men and most other animals have a certain organ (the mouth) for taking in nourishment; another (the food tube), for its digestion.

Because of its organization, each animal or plant is said to be an organism. Living things constitute the organic kingdom. Things without life and not formed by life constitute the inorganic, or mineral, kingdom. Mark I for inorganic and O for organic after the proper words in this list: granite, sugar, lumber, gold, shellac, sand, coal, paper, glass, starch, copper, gelatine, cloth, air, potatoes, alcohol, oil, clay. Which of these things are used for food by animals? Conclusion?

Energy in the Organic World.—We see animals exerting energy; that is, we see them moving about and doing work. Plants are never seen acting that way; yet they need energy in order to form their tissues, grow, and raise themselves in the air.

Source of Plant Energy.—We notice that green plants thrive only in the light, while animal growth is largely independent of light. In fact, in the salt mines of Poland there are churches and villages below the ground, and children are born, become adults, and live all their lives below ground, without seeing the sun. (That these people are not very strong is doubtless due more to want of fresh air and other causes than to want of sunlight.)

Fig. 1.—Surfaces of a Leaf, magnified.

Fig. 2.—A Leaf storing Energy in Sunlight.

The need of plants for sunlight shows that they must obtain something from the sun. This has been found to be energy. This enables them to lift their stems in growth, and form the various structures called tissues which make up their stems and leaves. It is noticed that they take in food and water from the soil through their roots. Experiments also show that green plants take in through pores (Fig. [1]), on the surface of their leaves, a gas composed of carbon and oxygen, and called carbon dioxide. The energy in the sunlight enables the plant to separate out the carbon, of the carbon dioxide and to build mineral and water and carbon into organic substances. The oxygen of the carbon dioxide is set free and returns to the air (Fig. [2]). Starch, sugar, oil, and woody fibre are examples of substances thus formed. Can you think of any fuel not due to plants?

How Animals obtain Energy.—You have noticed that starch, oil, etc., will burn, or oxidize, that is, unite with the oxygen of the air; thus the sun’s energy, stored in these substances, is changed back to heat and motion. The oxidation of oil or sugar may occur in a furnace; it may also occur in the living substance of the active animal.

Fortunately for the animals, the plants oxidize very little of the substances built up by them, since they do not move about nor need to keep themselves warm. We notice that animals are constantly using plant substances for food, and constantly drawing the air into their bodies. If the sunlight had not enabled the green plant to store up these substances and to set free the oxygen (Fig. [3]), animals would have no food to eat nor air to breathe; hence we may say that the sunlight is indirectly the source of the life and energy of animals. Mushrooms and other plants without green matter cannot set oxygen free (Fig. [3]).

Experiment to show the Cause of Burning, or Oxidation.—Obtain a large glass bottle (a pickle jar), a short candle, and some matches. Light the candle and put it on a table near the edge, and cover it with the glass jar. The flame slowly smothers and goes out. Why is this? Is the air now in the jar different from that which was in it before the candle was lighted? Some change must have taken place or the candle would continue to burn. To try whether the candle will burn again under the jar without changing the air, slide the jar to the edge of the table and let the candle drop out. Light the candle and slip it up into the jar again, the jar being held with its mouth a little over the edge of the table to receive the candle (Fig. [5]). The flame goes out at once. Evidently the air in the jar is not the same as the air outside. Take up the jar and wave it to and fro a few times, so as to remove the old air and admit fresh air. The candle now burns in it with as bright a flame as at first. So we conclude that the candle will not continue to burn unless there is a constant supply of fresh air. The gas formed by the burning is carbon dioxide. It is the gas from which plants extract carbon. (Beginners’ Botany, Chap. XIII.) One test for the presence of this gas is that it forms a white, chalky cloud in lime water; another is that it smothers a fire.

Experiment to show that Animals give off Carbon Dioxide.—Place a cardboard over the mouth of a bottle containing pure air. Take a long straw, the hollow stem of a weed, a glass tube, or a sheet of stiff paper rolled into a tube, and pass the tube into the bottle through a hole in the cardboard. Without drawing in a deep breath, send one long breath into the bottle through the tube, emptying the lungs by the breath as nearly as possible (Fig. [4]). Next, invert the bottle on the table as in the former experiment, afterward withdrawing the cardboard. Move the bottle to the edge of the table and pass the lighted candle up into it (Fig. [5]). Does the flame go out as quickly as in the former experiment?

If you breathe through a tube into clear lime water, the water turns milky. The effect of the breath on the candle and on the lime water shows that carbon dioxide is continually leaving our bodies in the breath.

Fig. 4.—Breathing into a bottle.

Fig. 5.—Testing the air in the bottle.

Oxidation and Deoxidation.—The union of oxygen with carbon and other substances, which occurs in fires and in the bodies of animals, is called oxidation. The separation of the oxygen from carbon such as occurs in the leaves of plants is called deoxidation. The first process sets energy free, the other process stores it up. Animals give off carbon dioxide from their lungs or gills, and plants give off oxygen from their leaves. But plants need some energy in growing, so oxidation also occurs in plants, but to a far less extent than in animals. At night, because of the absence of sunlight, no deoxidation is taking place in the plant, but oxidation and growth continue; so at night the plant actually breathes out some carbon dioxide. The deepest part of the lungs contains the most carbon dioxide. Why was it necessary to empty the lungs as nearly as possible in the experiment with the candle? Why would first drawing a deep breath interfere with the experiment? Why does closing the draught of a stove, thus shutting off part of the air, lessen the burning? Why does a “firefly” shine brighter at each breath? Why is the pulse and breathing faster in a fever? Very slow in a trance?

The key for understanding any animal is to find how it gets food and oxygen, and how it uses the energy thus obtained to grow, move, avoid its enemies, and get more food. Because it moves, it needs senses to guide it.

The key for understanding a plant is to find how it gets food and sunlight for its growth. It makes little provision against enemies; its food is in reach, so it needs no senses to guide it. The plant is built on the plan of having the nutritive activities near the surface (e.g. absorption by roots; gas exchange in leaves). The animal is built on the plan of having its nutritive activities on the inside (e.g. digestion; breathing).

Cell and Protoplasm.—Both plants and animals are composed of small parts called cells. Cells are usually microscopic in size. They have various shapes, as spherical, flat, cylindrical, fibre-like, star-shaped. The living substance of cells is called protoplasm. It is a stiff, gluey fluid, albuminous in its nature. Every cell has a denser spot or kernel called a nucleus, and in the nucleus is a still smaller speck called a nucleolus. Most cells are denser and tougher on the outside, and are said to have a cell wall, but many cells are naked, or without a wall. Hence the indispensable part of a cell is not the wall but the nucleus, and a cell may be defined as a bit of protoplasm containing a nucleus. This definition includes naked cells as well as cells with walls.

One-celled Animals.—There are countless millions of animals and plants the existence of which was not suspected until the invention of the microscope several centuries ago. They are one-celled, and hence microscopic in size. It is believed that the large animals and plants are descended from one-celled animals and plants. In fact, each individual plant or animal begins life as a single cell, called an egg cell, and forms its organs by the subdivision of the egg cell into many cells. An egg cell is shown in Fig. [6], and the first stages in the development of an egg cell are shown in Fig. [7].

Fig. 6.—Egg cell of mammal with yolk.

Fig. 7.—Egg cell subdivides into many cells forming a sphere (morula) containing a liquid. A dimple forms and deepens to form the next stage (gastrula).

The animals to be studied in the first chapter are one-celled animals. To understand them we must learn how they eat, breathe, feel, and move. They are called Protozoans (Greek protos, first, and zoon). All other animals are composed of many cells and are called Metazoans (Greek meta, beyond or after). The cells composing the mucous membrane in man are shown in Fig. [8]. The cellular structure of the leaf of a many-celled plant is illustrated in Fig. [1].

Method of Classifying Animals.—The various animals display differences more or less marked. The question arises, are not some of them more closely related than others? We conclude that they are, since the difference between some animals is very slight, while the difference between others is quite marked.

Fig. 8.—Mucous Membrane formed of one layer of cells. A few cells secrete mucus.

To show the different steps in classifying an animal, we will take an example,—the cow. Even little children learn to recognize a cow, although individual cows differ somewhat in form, size, colour, etc. The varieties of cows, such as short-horn, Jersey, etc., all form one species of animals, having the scientific name taurus. Let us include in a larger group the animals closest akin to a cow. We see a cat, a bison, and a dog; rejecting the cat and the dog, we see that the bison has horns, hoofs, and other similarities. We include it with the cow in a genus called Bos, calling the cow Bos taurus, and the bison, Bos bison. The sacred cow of India (Bos indicus) is so like the cow and the buffalo as also to belong to the genus Bos. Why is not the camel, which, like Bos bison, has a hump, placed in the genus Bos?

The Old World buffaloes,—most abundant in Africa and India,—the antelopes, sheep, goats, and several other genera are placed with the genus Bos in a family called the hollow-horned animals.

This family, because of its even number of toes and the habit of chewing the cud, resembles the camel family, the deer family, and several other families. These are all placed together in the next higher systematic unit called an order, in this case, the order of ruminants.

The ruminants, because they are covered with hair and nourish the young with milk, are in every essential respect related to the one-toed horses, the beasts of prey, the apes, etc. Hence they are all placed in a more inclusive division of animals, the class called mammals.

All mammals have the skeleton, or support of the body, on the inside, the axis of which is called the vertebral column. This feature also belongs to the classes of reptiles, amphibians, and fishes. It is therefore consistent to unite these classes by a general idea or conception into a great branch of animals called the vertebrates.

Returning from the general to the particular by successive steps, state the branch, class, order, family, genus, and species to which the cow belongs.

The Eight Branches or Sub-kingdoms.—The simplest classification divides the whole animal kingdom into eight branches, named and characterized as follows, beginning with the lowest: I. Protozoans. One-celled. II. Sponges. Many openings. III. Polyps. Circular; cuplike; having only one opening which is both mouth and vent. IV. Echinoderms. Circular; rough-skinned; two openings. V. Molluscs. No skeleton; usually with external shell. VI. Vermes. Elongate body, no jointed legs. VII. Arthropods. External jointed skeleton; jointed legs. VIII. Vertebrates. Internal jointed skeleton with axis or backbone.[^[1]]

[1]. This is the briefest classification. Animals have also been divided into twelve branches. The naming of animals is somewhat chaotic at present, but an attempt to come to an agreement is now being made by zoologists of all nations.

CHAPTER II
PROTOZOA (One-celled Animals)

The Amœba

Suggestions.—Amœbas live in the slime found on submerged stems and leaves in standing water, or in the ooze at the bottom. Water plants may be crowded into a glass dish and allowed to decay, and after about two weeks the amœba may be found in the brown slime scraped from the plants. An amœba culture sometimes lasts only three days. The most abundant supply ever used by the writer was from a bottle of water where some oats were germinating. Use ⅕ or ⅙ inch objective, and cover with a thin cover glass. Teachers who object to the use of the compound microscope in a first course should require a most careful study of the figures.

Fig. 9.—Amœba Proteus, much enlarged.

Fig. 10.—Amœba.
cv, contractile vacuole; ec, ectoplasm; en, endoplasm; n, nucleus; ps, pseudopod; ps, pseudopod forming; ectoplasm protrudes and endoplasm flows into it.

Form and Structure.—The amœba looks so much like a clear drop of jelly that a beginner cannot be certain that he has found one until it moves. It is a speck of protoplasm (Fig. [9]), with a clear outer layer, the ectoplasm; and a granular, internal part, the endoplasm. Is there a distinct line between them? (Fig. [10].)

Note the central portion and the slender prolongations or pseudopods (Greek, false feet). Does the endoplasm extend into the pseudopods? (Fig. [10].) Are the pseudopods arranged with any regularity?

Sometimes it is possible to see a denser appearing portion, called the nucleus; also a clear space, the contractile vacuole (Fig. [10]).

Fig. 11.—The same amœba seen at different times.

Movements.—Sometimes while the pseudopods are being extended and contracted, the central portion remains in the same place (this is motion). Usually only one pseudopod is extended, and the body flows into it; this is locomotion (Fig. [11]). There is a new foot made for each step.

Feeding.—If the amœba crawls near a food particle, the pseudopod is pressed against it, or a depression occurs (Fig. [12]), and the particle is soon embedded in the endoplasm. Often a clear space called a food vacuole is noticed around the food particle. This is the water that is taken in with the particle (Fig. [12]). The water and the particle are soon absorbed and assimilated by the endoplasm.

Fig. 12.—The Amœba taking food.

Excretion.—If a particle of sand or other indigestible matter is taken in, it is left behind as the amœba moves on. There is a clear space called the contractile vacuole, which slowly contracts and disappears, then reappears and expands (Figs. [9] and [10]). This possibly aids in excreting oxidized or useless material.

Circulation in the amœba consists of the movement of its protoplasmic particles. It lacks special organs of circulation.

Feeling.—Jarring the glass slide seems to be felt, for it causes the activity of the amœba to vary. It does not take in for food every particle that it touches. This may be the beginning of taste, based upon mere chemical affinity. The pseudopods aid in feeling.

Reproduction.—Sometimes an amœba is seen dividing into two parts. A narrowing takes place in the middle; the nucleus also divides, a part going to each portion (Fig. [13]). The mother amœba finally divides into two daughter amœbas. Sex is wanting.

Fig. 13.—Amœba, Dividing.

Source of the Amœba’s Energy.—We thus see that the amœba moves without feet, eats without a mouth, digests without a stomach, feels without nerves, and, it should also be stated, breathes without lungs, for oxygen is absorbed from the water by its whole surface. Its movements require energy; this, as in all animals, is furnished by the uniting of oxygen with the food. Carbon dioxide and other waste products are formed by the union; these pass off at the surface of the amœba and taint the water with impurities.

Questions.—Why will the amœba die in a very small quantity of water, even though the water contains enough food? Why will it die still quicker if air is excluded from contact with the drop of water?

The amœba never dies of old age. Can it be said to be immortal?

According to the definition of a cell (Chapter I), is the amœba a unicellular or multicellular animal?

Cysts.—If the water inhabited by a protozoan dries up, it encysts, that is, it forms a tough skin called a cyst. Upon return of better conditions it breaks the cyst and comes out. Encysted protozoans may be blown through the air: this explains their appearance in vessels of water containing suitable food but previously free from protozoans.

The Slipper Animalcule or Paramecium

Suggestions.—Stagnant water often contains the paramecium as well as the amœba; or they may be found in a dish of water containing hay or finely cut clover, after the dish has been allowed to stand in the sun for several days. A white film forming on the surface is a sign of their presence. They may even be seen with the unaided eye as tiny white particles by looking through the side of the dish or jar. Use at first a ⅓ or ¼ in. objective. Restrict their movements by placing cotton fibres beneath the cover glass; then examine with ⅕ or ⅙ objective. Otherwise, study figures.

Shape and Structure.—The paramecium’s whole body, like the amœba’s, is only one cell. It resembles a slipper in shape, but the pointed end is the hind end, the front end being rounded (Fig. [14]). The paramecium is propelled by the rapid beating of numerous fine, threadlike appendages on its surface, called cilia (Latin, eyelashes) (Figs.). The cilia, like the pseudopods of the amœba, are merely prolongations of the cell protoplasm, but they are permanent. The separation between the outer ectoplasm and the interior granular endoplasm is more marked than in the amœba (Fig. [14]).

Fig. 14.—Paramecium, showing cilia, c.
Two contractile vacuoles, cv; the macronucleus, mg; two micronuclei, mi; the gullet (Œ), a food ball forming and ten food balls in their course from gullet to vent, a.

Fig. 15.

Nucleus and Vacuoles.—There is a large nucleus called the macronucleus, and beside it a smaller one called the micronucleus. They are hard to see. About one third of the way from each end is a clear, pulsating space (bb. Fig. [15]) called the pulsating vacuole. These spaces contract until they disappear, and then reappear, gradually expanding. Tubes lead from the vacuoles which probably serve to keep the contents of the cell in circulation.

Fig. 16.—Two Paramecia exchanging parts of their nuclei.

Feeding.—A depression, or groove, is seen on one side; this serves as a mouth (Figs.). A tube which serves as a gullet leads from the mouth-groove to the interior of the cell. The mouth-groove is lined with cilia which sweep food particles inward. The particles accumulate in a mass at the inner end of the gullet, become separated from it as a food ball (Fig. [14]), and sink into the soft protoplasm of the body. The food balls follow a circular course through the endoplasm, keeping near the ectoplasm.

Fig. 17.—Vorticella (or bell animalcule), two extended, one withdrawn.

Reproduction.—This, as in the amœba, is by division, the constriction being in the middle, and part of the nucleus going to each half. Sometimes two individuals come together with their mouth-grooves touching and exchange parts of their nuclei (Fig. [16]). They then separate and each divides to form two new individuals.

Fig. 18.—Euglena.

We thus see that the paramecium, though of only one cell, is a much more complex and advanced animal than the amœba. The tiny paddles, or cilia, the mouth-groove, etc., have their special duties similar to the specialized organs of the many-celled animals to be studied later.

Fig. 19.—Shell of a Radiolarian.

If time and circumstances allow a prolonged study, several additional facts may be observed by the pupil, e.g. Does the paramecium swim with the same end always foremost, and same side uppermost? Can it move backwards? Avoid obstacles? Change shape in a narrow passage? Does refuse matter leave the body at any particular place? Trace movement of the food particles.

Draw the paramecium.

Which has more permanent parts, the amœba or paramecium? Name two anatomical similarities and three differences; four functional similarities and three differences.

The amœba belongs in the class of protozoans called Rhizopoda “root footed.”

Other classes of Protozoans are the Infusorians (in the broad sense of the term), which have many waving cilia (Fig. [17]) or one whiplike flagellum (Fig. [18]), and the Foraminifers, which possess a calcareous shell pierced with holes (Fig. [19]). Much chalky limestone has been formed of their shells. To which class does the paramecium belong?

Protozoans furnish a large amount of food to the higher animals.

CHAPTER III
SPONGES

Suggestions.—In many parts of North America, fresh-water sponges may, by careful searching, be found growing on rocks and logs in clear water. They are brown, creamy, or greenish in colour, and resemble more a cushion-like plant than an animal. They have a characteristic gritty feel. They soon die after removal to an aquarium.

Fig. 21.—Fresh-water Sponge.

A number of common small bath sponges may be bought and kept for use in studying the skeleton of an ocean sponge. These sponges should not have large holes in the bottom; if so, too much of the sponge has been cut away. A piece of marine sponge preserved in alcohol or formalin may be used for showing the sponge with its flesh in place. Microscopic slides may be used for showing the spicules.

Fig. 22.—Section of fresh-water sponge (enlarged).

The small fresh-water sponge (Fig. [21]) lacks the more or less vaselike form typical of sponges. It is a rounded mass growing upon a rock or a log. As indicated by the Arrows, where does water enter the sponge? This may be tested by putting colouring matter in the water near the living sponge. Where does the water come out? (Fig. [22].) Does it pass through ciliated chambers in its course? Is the surface of the sponge rough or smooth? Do any of the skeletal spicules show on the surface? (Fig. [21].) Does the sponge thin out near its edge?

Fig. 23.—Eggs and SPICULES of fresh-water sponge (enlarged).

The egg of this sponge is shown in Fig. [23]. It escapes from the parent sponge through the osculum, or large outlet. As in most sponges, the first stage after the egg is ciliated and free-swimming.

Marine Sponges.—The grantia (Fig. [24]) is one of the simplest of marine sponges. What is the shape of grantia? What is its length and diameter? How does the free end differ from the fixed end? Are the spicules projecting from its body few or many?

Fig. 24.—Grantia.

Where is the osculum, or large outlet? With what is this surrounded? The osculum opens from a central cavity called the cloaca. The canals from the pores lead to the cloaca.

Buds are sometimes seen growing out from the sponge near its base. These are young sponges formed asexually. Later they become detached from the parent sponge.

Fig. 25.—Plan of a sponge.

Commercial “Sponge.”—What part of the complete animal remains in the bath sponge? Slow growing sponges grow more at the top and form tall, simple, tubular or vaselike animals. Fast growing sponges grow on all sides at once and form a complicated system of canals, pores, and oscula. Which of these habits of growth do you think belonged to the bath sponge? Is there a large hole in the base of your specimen? If so, this is because the cloaca was reached in trimming the lower part where it was attached to a rock. Test the elasticity of the sponge when dry and when wet by squeezing it. Is it softer when wet or dry? Is it more elastic when wet or dry? How many oscula does your specimen have? How many inhalent pores to a square inch? Using a probe (a wire with knob at end, or small hat pin), try to trace the canals from the pores to the cavities inside.

Fig. 26.—Bath Sponge.

Do the fibres of the sponge appear to interlace, or join, according to any system? Do you see any fringe-like growths on the surface which show that new tubes are beginning to form? Was the sponge growing faster at the top, on the sides, or near the bottom?

Fig. 27.—Bath Sponge.

Fig. 28.—Bath Sponge.

Burn a bit of the sponge; from the odor, what would you judge of its composition? Is the inner cavity more conspicuous in a simple sponge or in a compound sponge like the bath sponge? Is the bath sponge branched or lobed? Compare a number of specimens (Figs. [26], [27], [28]) and decide whether the common sponge has a typical shape. What features do their forms possess in common?

Fig 29.—Skeleton of a glass sponge.

Sponges are divided into three classes, according as their skeletons are flinty (silicious), limy (calcareous), or horny.

Some of the silicious sponges have skeletons that resemble spun glass in their delicacy. Flint is chemically nearly the same as glass. The skeleton shown in Fig. [29] is that of a glass sponge which lives near the Philippine Islands.

The horny sponges do not have spicules in their skeletons, as the flinty and limy sponges have, but the skeleton is composed of interweaving fibres of spongin, a durable substance of the same chemical nature as silk (Figs. [30] and [31]).

The limy sponges have skeletons made of numerous spicules of lime. The three-rayed spicule is the commonest form.

Fig. 30.—A horny sponge.

Fig. 31.—Section of horny sponge.

The commercial sponge, seen as it grows in the ocean, appears as a roundish mass with a smooth, dark exterior, and having about the consistency of beef liver. Several large openings (oscula), from which the water flows, are visible on the upper surface. Smaller holes (inhalent pores—many of them so small as to be indistinguishable) are on the sides. If the sponge is disturbed, the smaller holes, and perhaps the larger ones, will close.

The outer layer of cells serves as a sort of skin. Since so much of the sponge is in contact with water, most of the cells do their own breathing, or absorption of oxygen and giving off of carbon dioxide. Nutriment is passed on from the surface cells to nourish the rest of the body.

Reproduction.—Egg cells and sperm-cells are produced by certain cells along the canals. The egg cell, after it is fertilized by the sperm cell, begins to divide and form new cells, some of which possess cilia. The embryo sponge passes out at an osculum. By the vibration of the cilia, it swims about for a while. It afterwards settles down with the one end attached to the ocean floor and remains fixed for the rest of its life. The other end develops oscula. Some of the cilia continue to vibrate and create currents which bring food and oxygen.

The cilia in many species are found only in cavities called ciliated chambers. (Figs. [22], [32].) There are no distinct organs in the sponge and there is very little specialization of cells. The ciliated cells and the reproductive cells are the only specialized cells. The sponges were for a long time considered as colonies of separate one-celled animals classed as protozoans. They are, without doubt, many-celled animals. If a living sponge is cut into pieces, each piece will grow and form a complete sponge.

Fig. 32.—Microscopic plan of ciliated chamber. Each cell lining the chamber has a nucleus, a whip-lash, and a collar around base of whip-lash. Question: State two uses of whip-lash.

That the sponge is not a colony of one-celled animals, each like an amœba, but is a many-celled animal, will be realized by examining Fig. [32], which shows a bit of sponge highly magnified. A sponge may be conceived as having developed from a one-celled animal as follows: Several one-celled animals happened to live side by side; each possessed a threadlike flagellum (E, Fig. [32]) or whip-lash for striking the water. By lashing the water, they caused a stronger current (Fig. [25]) than protozoans living singly could cause. Thus they obtained more food and multiplied more rapidly than those living alone. The habit of working together left its impress on the cells and was transmitted by inheritance.

Cell joined to cell formed a ring; ring joined to ring formed a tube which was still more effective than a ring in lashing the water into a current and bringing fresh food (particles of dead plants and animals) and oxygen.

Few animals eat sponges; possibly because spicules, or fibres, are found throughout the flesh, or because the taste and the odour are unpleasant enough to protect them. Small animals sometimes crawl into sponges to hide. One sponge grows upon shells inhabited by hermit crabs. Moving of the shell from place to place is an advantage to the sponge, while the sponge conceals and thus protects the crab.

Special Report: Sponge “Fisheries.” (Localities; how sponges are taken, cleaned, dried, shipped, and sold.)

CHAPTER IV
POLYPS (CUPLIKE ANIMALS)

The Hydra, or Fresh-Water Polyp

Fig. 33.—A Hydra.

Suggestions.—Except in the drier regions of North America the hydra can usually be found by careful search in fresh-water ponds not too stagnant. It is found attached to stones, sticks, or leaves, and has a slender, cylindrical body from a quarter to half an inch long, varying in thickness from that of a fine needle to that of a common pin. The green hydra and the brown hydra, both very small, are common species, though hydras are often white or colourless. They should be kept in a large glass dish filled with water. They may be distinguished by the naked eye but are not studied satisfactorily without a magnifying glass or microscope. Place a living specimen attached to a bit of wood in a watch crystal filled with water, or on a hollowed slip, or on a slip with a bit of weed to support the cover glass, and examine with hand lens or lowest power of microscope. Prepared microscopical sections, both transverse and longitudinal, may be bought of dealers in microscopic supplies. One is shown in Fig. [39].

Fig. 34.—Forms assumed by Hydra.

Is the hydra’s body round or two-sided? (Fig. [35].) What is its general shape? Does one individual keep the same shape? (Fig. [34].) How does the length of the threadlike tentacles compare with the length of the hydra’s body? About how many tentacles are on a hydra’s body? Do all have the same number of tentacles? Are the tentacles knotty or smooth? (Fig. [35].) The hydra is usually extended and slender; sometimes it is contracted and rounded. In which of these conditions is the base (the foot) larger around than the rest of the body? (Fig. [34].) Smaller? How many openings into the body are visible? Is there a depression or an eminence at the base of the tentacles? For what is the opening on top of the body probably used? Why are the tentacles placed at the top of the hydra’s body? Does the mouth have the most convenient location possible?

Fig. 35.—Hydra (much enlarged).

The conical projection bearing the mouth is called hypostome (Fig. [34]). The mouth opens into the digestive cavity. Is this the same as the general body cavity, or does the stomach have a wall distinct from the body cavity? How far down does the body cavity extend? Does it extend up into the tentacles? (Fig. [39].)

If a tentacle is touched, what happens? Is the body ever bent? Which is more sensitive, the columnar body or the tentacles? In searching for hydras would you be more likely to find the tentacles extended or drawn in? Is the hypostome ever extended or drawn in? (Fig. [34].)

Locomotion.—The round surface, or disk, by which the hydra is attached, is called its foot. Can you move on one foot without hopping? The hydra moves by alternately elongating and rounding the foot. Can you discover other ways by which it moves? Does the hydra always stand upon its foot?

Fig. 36.—Nettling Cell. II. discharged, and I. not discharged.

Lasso Cells.—Upon the tentacles (Fig. [35]) are numerous cells provided each with a threadlike process (Fig. [36]) which lies coiled within the cell, but which may be thrown out upon a water flea, or other minute animal that comes in reach. The touch of the lasso paralyzes the prey (Fig. [37]). These cells are variously called lasso cells, nettling cells, or thread cells. The thread is hollow and is pushed out by the pressure of liquid within. When the pressure is withdrawn the thread goes back as the finger of a glove may be turned back into the glove by turning the finger outside in. When a minute animal, or other particle of food comes in contact with a tentacle, how does the tentacle get the food to the mouth? By bending and bringing the end to the mouth, or by shortening and changing its form, or in both ways? (Fig. [34], C.) Do the neighbouring tentacles seem to bend over to assist a tentacle in securing prey? (Fig. [34], C.)

Fig. 37.—Hydra capturing a water flea.

Digestion.—The food particles break up before remaining long in the stomach, and the nutritive part is absorbed by the lining cells, or endoderm (Fig. [39]). The indigestible remnants go out through the mouth. The hydra is not provided with a special vent. Why could the vent not be situated at the end opposite the mouth?

Fig. 38.—Hydras on the under surface of pondweed.

Circulation and Respiration.—Does water have free access to the body cavity? Does the hydra have few or nearly all of its cells exposed to the water in which it lives? From its structure, decide whether it can breathe like a sponge or whether special respiratory cells are necessary to supply it with oxygen and give off carbon dioxide. Blood vessels are unnecessary for transferring oxygen and food from cell to cell.

Reproduction.—Do you see any swellings upon the side of the hydra? (Fig. [34], A.) If the swelling is near the tentacles, it is a spermary; if near the base, it is an ovary. A sperm coalesces with or fertilizes the ovum after the ovum is exposed by the breaking of the ovary wall. Sometimes the sperm from one hydra unites with the ovum of another hydra. This is called cross-fertilization. The same term is applied to the process in plants when the male element, developed in the pollen of the flower, unites with the female element of the ovule of the flower on another plant. The hydra, like most plants and some other animals, is hermaphrodite, that is to say, both sperms and ova are produced by one individual. In the autumn, eggs are produced with hard shells to withstand the cold until spring. Sexual reproduction takes place when food is scarce. Asexual generation (by budding) is common with the hydra when food supply is abundant. After the bud grows to a certain size, the outer layer of cells at the base of the bud constricts and the young hydra is detached.

Fig. 39.—Longitudinal section of hydra (microscopic and diagrammatic).

Compare the sponge and the hydra in the following respects:—many celled, or one-celled; obtaining food; breathing; tubes and cavities; openings; reproduction; locomotion. Which ranks higher among the metazoa? The metazoa, or many-celled animals, include all animals except which branch?

Figure 39 is a microscopic view of a vertical section of a hydra to show the structure of the body wall. There is an outer layer called the ectoderm, and an inner layer called the endoderm. There is also a thin supporting layer (black in the figure) called the mesoglea. The mesoglea is the thinnest layer. Are the cells larger in the endoderm or the ectoderm? Do both layers of cells assist in forming the reproductive bud? The ectoderm cells end on the inside in contractile tails which form a thin line and have the effect of muscle fibres. They serve the hydra for its remarkable changes of shape. When the hydra is cut in pieces, each piece makes a complete hydra, provided it contains both endoderm and ectoderm.

In what ways does the hydra show “division of labour”? Answer this by explaining the classes of cells specialized to serve a different purpose. Which cells of the hydra are least specialized? In what particulars is the plan of the hydra different from that of a simple sponge? An ingenious naturalist living more than a century ago, asserted that it made no difference to the hydra whether the ectoderm or the endoderm layer were outside or inside,—that it could digest equally well with either layer. He allowed a hydra to swallow a worm attached to a thread, and then by gently pulling in the thread, turned the hydra inside out. More recently a Japanese naturalist showed that the hydra could easily be turned inside out, but he also found that when left to itself it soon reversed matters and returned to its natural condition, that the cells are really specialized and each layer can do its own work and no other.

Habits.—The hydra’s whole body is a hollow bag, the cavity extending even into the tentacles. The tentacles may increase in number as the hydra grows but seldom exceed eight. The hydra has more active motion than locomotion. It seldom moves from its place, but its tentacles are constantly bending, straightening, contracting, and expanding. The body is also usually in motion, bending from one side to another. When the tentacles approach the mouth with captured prey, the mouth (invisible without a hand lens) opens widely, showing five lobes or lips, and the booty is soon tucked within. A hydra can swallow an animal larger in diameter than itself.

The endoderm cells have amœboid motion, that is, they extend pseudopods. They also resemble amœbas in the power of intra-cellular digestion; that is, they absorb the harder particles of food and digest them afterwards, rejecting the indigestible portions. Some of these cells have flagella (see Fig. [39]) which keep the fluid of the cavity in constant motion.

Sometimes the hydra moves after the manner of a small caterpillar called a “measuring worm,” that is, it takes hold first by the foot, then by the tentacles, looping its body at each step. Sometimes the body goes end over end in slow somersaults.

Fig. 40.—Hydroid Colony, with nutritive (P) reproductive (M) and defensive (S) hydranths.

The length of the extended hydra may reach one half inch. When touched, both tentacles and body contract until it looks to the unaided eye like a round speck of jelly. This shows sensibility, and a few small star-shaped cells are believed to be nerve cells, but the hydra has not a nervous system. Hydras show their liking for light by moving to the side of the vessel or aquarium whence the light comes.

Fig. 41.—“Portuguese Man-o’-war” (compare with Fig. [40]). A floating hydroid colony with long, stinging (and sensory) streamers. Troublesome to bathers in Gulf of Mexico. Notice balloon-like float.

The Branch Polyps (sometimes called Cœlenterata).—The hydra is the chief fresh-water representative of this great branch of the animal kingdom. This branch is characterized by its members having only one opening to the body. The polyps also include the salt water animals called hydroids, jellyfishes, and coral polyps.

Hydroids.—Figure 40 shows a hydroid, or group of hydra-like growths, one of which eats and digests for the group, another defends by nettling cells, another produces eggs. Each hydra-like part of a hydroid is called a hydranth. Sometimes the buds on the hydra remain attached so long that a bud forms upon the first bud. Thus three generations are represented in one organism. Such growths show us that it is not always easy to tell what constitutes an individual animal.

Fig. 42.—The formation of many free-swimming jellyfishes from one fixed hydra-like form. The saucer-like parts (h) turn over after they separate and become like Fig. [43] or 44. Letters show sequence of diagrams.

Hydroids may be conceived to have been developed by the failure of budding hydras to separate from the parent, and by the gradual formation of the habit of living together and assisting one another. When each hydranth of the hydroid devoted itself to a special function of digestion, defence, or reproduction, this group lived longer and prospered; more eggs were formed, and the habits of the group were transmitted to a more numerous progeny than were the habits of a group where members worked more independently of one another.

As the sponge is a simple example of the devotion of special cells to special purposes, the hydroid is a primitive and simple example of the occurrence of organs, that is of special parts of the body set aside for a special work.

Fig. 43.—A Jellyfish.

How many mature hydranths are seen in the hydroid shown in Fig. [40]? Why are the defensive hydranths on the outside of the colony? Which hydranths have no tentacles? Why not?

Fig. 44.—A Jellyfish (medusa).

Jellyfish.—Alternation of Generations.—Medusa.—With some species of hydroids, a very curious thing happens.—The hydranth that is to produce the eggs falls off and becomes independent of the colony. More surprising still, its appearance changes entirely and instead of being hydra-like, it becomes the large and complex creature called jellyfish (Fig. [43]). But the egg of the jellyfish produces a small hydra-like animal which gives rise by budding to a hydroid, and the cycle is complete.

The bud (or reproductive hydranth) of the hydroid does not produce a hydroid, but a jellyfish; the egg of the jellyfish does not produce a jellyfish, but a hydroid. This is called by zoologists, alternation of generations. A complete individual is the life from the germination of one egg to the production of another. So that an “individual” consists of a hydroid colony fixed in one place together with all the jellyfish produced from its buds, which may now be floating miles away from it in the ocean. Bathers in the surf are sometimes touched and stung by the long, streamer-like tentacles of the jellyfish. These, like the tentacles of the hydra, have nettling cells (Fig. [41]).

Fig. 45.—Coral Polyps (tentacles, a multiple of six). Notice hypostome.

The umbrella-shaped free-swimming jellyfish is called a medusa (Fig. [44]).

Coral Polyps.—Some of the salt water relatives of the hydra produce buds which remain attached to the parent without, however, becoming different from the parent in any way. The coral polyps and corallines are examples of colonies of this kind, possessing a common stalk which is formed as the process of multiplication goes on. In the case of coral polyps, the separate animals and the flesh connecting them secrete within themselves a hard, limy, supporting structure known as coral. In some species, the coral, or stony part, is so developed that the polyp seems to be inserted in the coral, into which it withdraws itself for partial protection (Fig. [45]).

The corallines secrete a smooth stalk which affords no protection, but they also secrete a coating or sheath which incloses both themselves and the stalk. The coating has apertures through which the polyps protrude in order to feed when no danger is near (Fig. [46]). The red “corals” used for jewelry are bits of stalks of corallines. The corallines (Figs. [47], [48]) are not so abundant nor so important as the coral polyps (Figs. [45], [49]).

Fig. 46.—Red Coralline with crust and polyps (eight tentacles).

Fig. 47.—Sea Fan (a coralline).

Fig. 48.—Organ Pipe “Coral” (a coralline).

Colonies of coral polyps grow in countless numbers in the tropical seas. The coral formed by successive colonies of polyps accumulates and builds up many islands and important additions to continents. The Florida “keys,” or islands, and the southern part of the mainland of Florida were so formed.

Fig. 49.—Upright cut through coral polyp × 4.
ms, mouth; mr, gullet; ls, ls, fleshy partitions (mesenteries) extending from outer body wall to gullet (to increase absorbing surface); s, s, shorter partitions; mb, fb, stony support (of lime, called coral); t, tentacles.

Fig. 50.—Sea Anemone.

The Sea Anemone, like the coral polyp, lives in the sea, but like the fresh-water hydra, it deposits no limy support for its body. The anemone is much larger than the hydra and most coral polyps, many species attaining a height of several inches. It does not form colonies. When its arms are drawn in, it looks like a large knob of shiny but opaque jelly. Polyps used to be called zoophytes (plant-animals), because of their flower-like appearance (Figs. [50], [51]).

Fig. 51.—Sea Anemones.

CHAPTER V
ECHINODERMS (SPINY ANIMALS)

The Starfish

Fig. 52.—Starfish on a rocky shore.

Suggestions. Since the echinoderms are aberrant though interesting forms not in the regular line of development of animals, this chapter may be omitted if it is desired to shorten the course.—The common starfish occurs along the Atlantic coast. It is captured by wading along the shore when the tide is out. It is killed by immersion in warm, fresh water. Specimens are usually preserved in 4 per cent formalin. Dried starfish and sea urchins are also useful. A living starfish kept in a pail of salt water will be instructive.

Fig. 53.—Plan of starfish; III, madreporite.

External Features.—Starfish are usually brown or yellow. Why? (See Fig. [52].) Has it a head or a tail? Right and left sides? What is the shape of the disk, or part which bears the five arms or rays? (Fig. [53].) Does the body as a whole have symmetry on two sides of a line (bilateral symmetry), or around a point (radial symmetry)? Do the separate rays have bilateral symmetry? The skeleton consists of limy plates embedded in the tough skin (Fig. [54]). Is the skin rough or smooth? Hard or soft? Are the projections (or spines) in the skin long or short? The skin is hardened by the limy plates, except around the mouth, which is at the centre of the lower side and surrounded by a membrane. Which is rougher, the mouth side, (oral side) or the opposite (aboral side)? Which side is more nearly flat? The vent is at or near the centre of the disk on the aboral surface. It is usually very small and sometimes absent. Why a vent is not of much use will be understood after learning how the starfish takes food.

Fig. 54.—Limy Plates in portion of a ray.

Fig. 55.—Starfish (showing Madreporite).

Fig. 56.—Water tube System of starfish.
m, madreporite; stc, stone canal; ap, ampulla.

An organ peculiar to animals of this branch, and called the madreporic plate, or madreporite, is found on the aboral surface between the bases of two rays (Fig. [55]). It is wartlike, and usually white or red. This plate is a sieve; the small openings keep out sand but allow water to filter through.

Movements: the Water-tube System.—The water, which is filtered through the perforated madreporite, is needed to supply a system of canals (Fig. [56]). The madreporite opens into a canal called the stone canal, the wall of which is hardened by the same kind of material as that found in the skin. The stone canal leads to the ring canal which surrounds the mouth (Fig. [56]). The ring canal sends radial canals into each ray to supply the double row of tube feet found in the groove at the lower side of each ray (Fig. [57]). Because of their arrangement in rows, the feet are also called ambulacral feet (Latin ambulacra, “forest walks”). There is a water holder (ampulla), or muscular water bulb at the base of each tube foot (Fig. [58]). These contract and force the water into the tube feet and extend them. The cuplike ends of the tubes cling to the ground by suction. The feet contain delicate muscles by which they contract and shorten. Thus the animal pulls itself slowly along, hundreds of feet acting together. The tube feet, for their own protection, may contract and retire into the groove, the water which extended them being sent back into the ampulla. This system of water vessels (or water-vascular system) of the echinodermata is characteristic of them; i.e. is not found elsewhere in the animal kingdom. The grooves and the plates on each side of them occupy the ambulacral areas. The rows of spines on each side of the grooves are freely movable. (What advantage?) The spines on the aboral surface are not freely movable.

Fig. 57.—Starfish, from below; tube feet extended.

Fig. 58.—Section of one ray and central portion of starfish.
f₁, f₂, f₃, tube feet more or less extended; au, eye spot; k, gills; da, stomach; m, madreporite; st, stone canal; p, ampulla; ei, ovary.

Respiration.—The system of water vessels serves the additional purpose of bringing water containing oxygen into contact with various parts of the body, and the starfish was formerly thought to have no special respiratory organs. However, there are holes in the aboral wall through which the folds of the delicate lining membrane protrude. These are now supposed to be gills (k, Fig. [58]).

Fig. 59.—Starfish eating a sea snail.
b, stomach everted.

The nervous system is so close to the aboral surface that much of it is visible without dissection. Its chief parts are a nerve ring around the mouth, which sends off a branch along each ray. These branches may be seen by separating the rows of tube feet. They end in a pigmented cell at the end of each ray called the eye-spot.

The food of starfish consists of such animals as crabs, snails, and oysters. When the prey is too large to be taken into the mouth, the starfish turns its stomach inside out over the prey (Fig. [59]). After the shells separate, the stomach is applied to the soft digestible parts. After the animal is eaten, the stomach is retracted. This odd way of eating is very economical to its digestive powers, for only that part of the food which can be digested and absorbed is taken into the body. Only the lower part of the stomach is wide and extensible. The upper portion (next to the aboral surface) is not so wide. This portion receives the secretion from five pairs of digestive glands, a pair of which is situated in each ray. Jaws and teeth are absent. (Why?) The vent is sometimes wanting. Why?

Reproduction.—There is a pair of ovaries at the base of each ray of the female starfish (Fig. [58]). The spermaries of the male have the same position and form as the ovaries, but they are of a lighter colour, usually white.[^[2]]

[2]. The sperm cells and egg cells are poured out into the water by the adults, and the sperm cell, which, like nearly all sperm cells, has a vibratory, taillike flagellum to propel it, reaches and fertilizes the egg cell.

Regeneration after Mutilation.—If a starfish loses one or more rays, they are replaced by growth. Only a very ignorant oysterman, angry at the depredations of starfish upon his oyster beds, would chop starfish to pieces, as this only serves to multiply them. This power simulates multiplication by division in the simplest animals.

Fig. 60.—Young starfish crawling upon their mother. (Challenger Reports.)

Steps in Advance of Lower Branches.—The starfish and other echinodermata have a more developed nervous system, sensory organs, and digestion, than forms previously studied; most distinctive of all, they have a body cavity distinct from the food cavity. The digestive glands, reproductive glands, and the fluid which serves imperfectly for blood, are in the body cavity. There is no heart or blood vessels. The motions of the stomach and the bending of the rays give motion to this fluid in the body cavity. It cannot be called blood, but it contains white blood corpuscles.

The starfish when first hatched is an actively swimming bilateral animal, but it soon becomes starlike (Fig. [60]). The limy plates of the starfish belong neither to the outer nor to the inner layer (endoderm and ectoderm) of the body wall, but to a third or middle layer (mesoderm); for echinoderms, like the polyps, belong to the three-layered animals. In this its skeleton differs from the shell of a crawfish, which is formed by the hardening of the skin itself.

Protective Coloration.—Many starfish are brown or yellow. This makes them inconspicuous on the brown rocks or yellow sand. Brightly coloured species are usually chosen for aquaria.

The Sea Urchin

External Features.—What is the shape of the body? What kind of symmetry has it? Do you find the oral (or mouth) surface? The aboral surface? Where is the body flattened? What is the shape of the spines? What is their use? How are the tube feet arranged? Where do the rows begin and end? Would you think that a sea urchin placed upside down in water, could right itself less or more readily than a starfish? What advantage in turning would each have that the other would not have? The name sea urchin has no reference to a mischievous boy, but means sea hedgehog (French oursin, hedgehog), the name being suggested by its spines.

Fig. 61.—A Sea Urchin crawling up the glass front wall of an aquarium (showing mouth spines and tube feet).

Comparison of Starfish and Sea Urchin.—The water system of the sea urchin, consisting of madreporite, tubes, and water bulbs, or ampullæ, is similar to that of the starfish. The tube feet and locomotion are alike. There is no need for well-developed respiratory organs in either animal, as the whole body, inside and out, is bathed in water. The method of reproduction is the same.

Fig. 62.—A Sea Urchin with spines removed, the limy plates showing the knobs on which the spines grew.

Fig. 63.—Section of Sea Urchin with soft parts removed, showing the jaws which bear the teeth protruding in Fig. [62].

The starfish eats soft animal food. The food of the sea urchin is mainly vegetable, and it needs teeth (Fig. [62], [63]); its food tube is longer than that of a starfish, just as the food tube of a sheep, whose food digests slowly, is much longer than that of a dog.

Fig. 64.—The Sea Otter, an urchin with mouth (o) and vent (A) on same side of body.

The largest species of sea urchins are almost as big as a child’s head, but such size is unusual. The spines are mounted on knobs, and the joint resembles a ball-and-socket joint, and allows a wide range of movement. Some sea urchins live on sandy shores, other species live upon the rocks. The sand dollars are of a lighter colour. (Why)? They are usually flatter and have lighter, thinner walls than the other species. The five-holed sand cake or sand dollar has its weight still further diminished by the holes, which also allow it to rise more easily through the water.

Both starfish and sea urchin rest on the flattened lower surface of the body, while the tube feet are stretching forward for another step.

Other Echinoderms

Fig. 65.—Sea Cucumbers.

The sea cucumbers, or holothurians, resemble the sea urchin in many respects, but their bodies are elongated, and the limy plates are absent or very minute. The mouth is surrounded by tentacles (Fig. [65]).

Fig. 66.—A Brittle Star.

The brittle stars resemble the starfish in form, but their rays are very slender, more distinct from the disk, and the tube feet are on the edges of the rays, not under them (Fig. [66]).

Fig. 67.—Crinoid, arms closed.

Fig. 68.—Disk of Crinoid from above, showing mouth in centre and vent near it, at right (arms removed).

The crinoids are the most ancient of the echinoderms. (Figs. [67], [68].) Their fossils are very abundant in the rocks. They inhabited the geological seas, and it is believed that some of the other echinoderms descended from them. A few now inhabit the deep seas. Some species are fixed by stems when young, and later break away and become free-swimming, others remain fixed throughout life.

The four classes of the branch echinoderms are Starfish (asteroids), Sea urchins (echinoids), Sea cucumbers (holothurians), and Sea lilies (crinoids).

Comparative Review

Make a table like this as large as the page of the notebook will allow, and fill in without guessing.

CHAPTER VI
WORMS

Suggestions:—Earthworms may be found in the daytime after a heavy rain, or by digging or turning over planks, logs, etc., in damp places. They may be found on the surface at night by searching with a lantern. Live specimens may be kept in the laboratory in a box packed with damp (not wet) loam and dead leaves. They may be fed on bits of fat meat, cabbage, onion, etc., dropped on the surface. When studying live worms, they should be allowed to crawl on damp paper or wood. An earthworm placed in a glass tube with rich, damp soil, may be watched from day to day.

Fig. 69.—An Earthworm.

External Features.—Is the body bilateral? Is there a dorsal and a ventral surface? Can you show this by a test with live worm? Do you know of an animal with dorsal and ventral surface, but not bilateral?

Can you make out a head? A head end? A neck? Touch the head and test whether it can be made to crawl backwards. Which end is more tapering? Is the mouth at the tip of the head end or on the upper or lower surface? How is the vent situated? Its shape? As the worm lies on a horizontal surface, is the body anywhere flattened? Are there any very distinct divisions in the body? Do you see any eyes?

Experiment to find whether the worm is sensitive (1) to touch, (2) to light, (3) to strong odours, (4) to irritating liquids. Does it show a sense of taste? The experiments should show whether it avoids or seeks a bright light, as of a window; also whether any parts of the body are especially sensitive to touch, or all equally sensitive. What effect when a bright light is brought suddenly near it at night?

Is red blood visible through the skin? Can you notice any pulsations in a vessel along the back? Do all earthworms have the same number of divisions or rings? Compare the size of the rings or segments. Can it crawl faster on glass or on paper?

Fig. 70.—Mouth and Setæ.

A magnifying glass will show on most species tiny bristle-like projections called setæ. How are the setæ arranged? (d, Fig. [70].) How many on one ring of the worm? How do they point? Does the worm feel smoother when it is pulled forward or backward between the fingers? Why? Are setæ on the lower surface? Upper surface? The sides? What is the use of the setæ? Are they useful below ground? Does the worm move at a uniform rate? What change in form occurs as the front part of the body is pushed forward? As the hinder part is pulled onward? How far does it go at each movement? At certain seasons a broad band, or ring, appears, covering several segments and making them seem enlarged (Fig. [71]). This is the clitellum, or reproductive girdle. Is this girdle nearer the mouth or the tail?