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THE LIFE OF CRUSTACEA

BY

W. T. CALMAN, D.Sc.

WITH THIRTY-TWO PLATES AND EIGHTY-FIVE FIGURES

METHUEN & CO. LTD.
36 ESSEX STREET W.C.
LONDON

[First Published in 1911]

[PREFACE]

This sketch of the Natural History of the Crustacea deals chiefly with their habits and modes of life, and attempts to provide, for readers unfamiliar with the technicalities of Zoology, an account of some of the more important scientific problems suggested by a study of the living animals in relation to their environment.

I am indebted to the Trustees of the British Museum for leave to reproduce certain figures prepared for the "Guide to the Crustacea, Arachnida, Onychophora, and Myriopoda exhibited in the Department of Zoology"; also to Sir Ray Lankester, K.C.B., F.R.S., and to Messrs. A. and C. Black for the use of a number of figures from my volume on Crustacea in the "Treatise on Zoology," edited by Sir Ray Lankester.

The source of these figures is indicated in the explanation attached to each. Of the remaining illustrations, some are reproduced from photographs of specimens in the collection of the British Museum; the others have been drawn from Nature, or copied from the original figures of various authors, by Miss Gertrude M. Woodward, to whom I am much indebted for the care and skill which she has given to their preparation.

W. T. C.

[CONTENTS]

[LIST OF ILLUSTRATIONS IN THE TEXT]

[FULL-PAGE PLATES]

THE LIFE OF CRUSTACEA

[CHAPTER I]

INTRODUCTORY

Everyone 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, and the most unobservant of seaside visitors must have had his attention attracted by living specimens of some of the more obtrusive species, such as the common Shore Crab. Many, however, will be surprised to learn that the Barnacles coating the rocks on the seashore, the Sand-hoppers of the beach, and the Woodlice of our gardens, are members of the same class. Still less is it suspected, by those who have not given special attention to the subject, that the living species of the group number many thousands, presenting strange diversities of structure and habits, and playing important parts in the general economy of Nature.

In addition to those just mentioned, a few Crustacea are sufficiently well known to be distinguished by popular names, such, for example, as Crayfish and Hermit Crabs, but for the vast majority no names are available except those of technical zoology. In the following pages, therefore, while technical terms have been introduced as sparingly as possible, the unfamiliarity of the animals themselves makes it needful to use many unfamiliar names.

In the classification of the Animal Kingdom, the Crustacea form one of the divisions of a comprehensive group, or Phylum, known as Arthropoda. The typical members of this group have a more or less firm external skeleton, the body is divided into segments, there are jointed limbs, and some of these are modified to serve as jaws. The chief divisions or classes of the Arthropoda are—(i.) Insecta, including Butterflies, Moths, Bees, Wasps, Flies, Beetles, and the like; (ii.) Chilopoda, or Centipedes; (iii.) Diplopoda, or Millipedes[1]; (iv.) Onychophora, including the curious worm-like Peripatus; (v.) Arachnida, or Scorpions, Spiders, Mites, and their allies; and (vi.) Crustacea.

It is not easy to summarize in a few words the characters common to all Crustacea, and distinguishing them from the other groups of Arthropoda. As a rough guide to classification, it is useful to remember that an Insect can generally be recognized by having three pairs of walking legs, an Arachnid by having four pairs, and a Centipede or Millipede by having a great many pairs, all nearly alike. The Crustacea, on the other hand, show great diversity in the number and arrangement of their walking or swimming legs, but they rarely show any special resemblance to those of the other large groups of Arthropoda. Thus, for example, a common species of Woodlouse, Armadillidium vulgare, is very similar at first sight to the Millipede Glomeris marginata, but it has only seven pairs of walking legs, while the Millipede has seventeen or nineteen pairs.

More precisely, it may be said of the great majority of Crustacea that they 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. Exceptions to each of these statements will be mentioned in later chapters in dealing with parasites and other highly modified types. In such cases, however, the larval or young stages afford indications of affinity, and comparison with less modified forms enables us to trace a connection with the typical Crustacea.

The best way to form a conception of a group of animals, however, is not to attempt in the first place to define its limits, but to begin by studying the structure of some typical and central species, and afterwards to note the divergences from this type presented by other members of the group. Speaking very generally, it may be said that these divergences are of two kinds. On the one hand there are characters that have no apparent relation to the animal's habits and mode of life, and on the other hand there are modifications of structure which are more or less plainly of use to the animal. It is to characters of the former class that we look for evidence of an animal's affinities, and it is upon them that our systems of classification are chiefly based. The characters of the second class—"adaptive" characters, as they are called—become of importance when we study the animal "as a going concern," so to speak, and endeavour to understand how its life is carried on in relation to its surroundings.

In pursuance of this plan of study, the next chapter will be devoted to a description of the Common Lobster as a type of the Crustacea. In the third chapter a survey of the classification of the group will be given; since, however, the characters on which the classification is based cannot be explained fully without entering into technical details which are beyond the scope of this work, this survey will be restricted to what is necessary for comprehension of the succeeding chapters. In the fourth chapter some account is given of the young or larval stages of Crustacea, and of the changes they undergo in the course of development.

In the next five chapters the Crustacea are classified according to their habitats, and those living in the shallow waters, the depths, and the surface of the ocean, in the fresh waters, and on land, are discussed in turn; while a separate chapter is devoted to the curious forms that live as parasites on, or as associates with, other animals. The last two chapters deal respectively with the Crustacea as they affect man, and with the past history of the group as revealed by fossil remains.

[CHAPTER II]

THE LOBSTER AS A TYPE OF CRUSTACEA

The most noticeable feature distinguishing the Lobster[2] ([Fig. 1]) at first sight from other familiar animals is the jointed shelly armour that encases its body and limbs. Over the fore part of the body this armour is continuous, forming a shield, or carapace, which projects in front, between the eyes, as a toothed beak, or rostrum; on the hinder part—the tail, or abdomen—it is divided into six segments, or somites, connected with each other by movable joints. Each of these somites carries on the under-side a pair of fin-like limbs, or swimmerets, the last pair of which (uropods) are much larger than the others, and are spread out at the sides of a middle tail-plate, or telson, forming what is known as the tail-fan. Since the fore part of the body also has a series of paired limbs, constructed, as will be shown later, on the same plan as the swimmerets, it is concluded that this part also is built up of somites, which have become soldered together. That this conclusion is correct is shown by comparison with some of the lower Crustacea in which this part of the body is divided up into eight separate somites, like those of the abdomen, each carrying, in place of the swimmerets, a pair of walking legs. In front of these eight somites, forming what is called the thorax, is the head—a part of the body which is never, in any Crustacean, broken up into distinct somites, but which, since it carries five pairs of appendages, must consist of at least five somites. The part of the body covered by the Lobster's carapace includes both the head and the thorax, and is known, therefore, as the cephalothorax. It is necessary to bear in mind that the parts of the body to which the names head, thorax, and abdomen, are applied in Crustacea are by no means exactly equivalent to those which bear the same names in Insects, for example, and that, beyond a rough similarity in position, they have no sort of relation to the parts so named in the body of a vertebrate animal.

Fig. 1—The Common Lobster (Homarus gammarus,) Female, from the Side. (From British Museum Guide.)

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Fig. 2—One of the Abdominal Somites of the Lobster, with its Appendages, separated and viewed from in Front. (From British Museum Guide.)

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There are altogether twenty pairs of appendages attached to the body of the Lobster. In front of the head are the stalked eyes (of which the nature will be discussed later) and two pairs of feelers—the antennules and antennæ (sometimes called the first and second antennæ). Near the mouth on the under-side of the head are three pairs of jaw-appendages—the strong mandibles and the flattened, leaf-like maxillulæ and maxillæ. Following these are the appendages of the thorax, of which the first three are intermediate in form between the true jaws and the legs, and are therefore termed foot-jaws, or maxillipeds. The remaining five pairs of thoracic limbs are the legs, the first pair forming the large and powerful pincer-claws, or chelipeds, while the others are the walking legs. The six pairs of swimmerets on the abdomen have already been mentioned.

If one of the somites of the abdomen be separated from the others, it will be seen ([Fig. 2]) to consist of a shelly ring, to which the two swimmerets are attached, wide apart, on the under-side. The arched upper part of the ring is known as the tergum, and the more flattened under-part as the sternum. On each side the tergum overlaps the sternum, and hangs down as a side-flap, or pleuron. On the upper side of the abdomen the terga of the somites overlap, the front part of each being pushed under the tergum in front when the abdomen is straightened, and only exposed to view when the abdomen is bent. Below, the sternum of each somite is seen to be only a narrow bar, connected with those in front and behind by soft membrane, and there is no overlapping. At the sides the somites are connected together by hinge-joints, which allow them to move only in a vertical plane. Thus the abdomen can be straightened out or bent downwards and forwards, but cannot be moved from side to side. In life the Lobster can swim backwards through the water by vigorously flapping the abdomen.

The carapace which covers the upper side of the head and thorax is not formed, as might be thought, simply by the terga of the somites becoming soldered together. This is shown by a comparison with certain shrimp-like Crustacea (Mysidacea) in which the carapace arises, like a fold of the skin, from the hinder edge of the head, and envelops, like a loose jacket, the distinctly segmented thorax. In the Lobster this fold has become adherent to the thoracic somites down the middle of the back, but at the sides it hangs free, enclosing on each side a cavity within which lie the gills.

It seems at first sight strange to include in the same category as "limbs" or "appendages" organs which differ so much in form and function as do the swimmerets, the walking legs, the jaws, and the antennæ. Nevertheless it can easily be demonstrated that all of them are constructed on the same general plan, and arise in the embryo from rudiments which are, for the most part, exactly alike. This is expressed in technical language by saying that the appendages of the whole series are homologous with one another. A full discussion of this interesting fact would require more space than can be devoted to it here, but a few examples may be given to illustrate what is meant by the "serial homology" of the appendages in Crustacea.

Fig. 3—Third Maxilliped of Lobster. (From British Museum Guide.)

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If one of the swimmerets be detached from the third abdominal somite, it will be seen ([Fig. 2]) to consist of a stalk, known as the protopodite, bearing two branches, of which that on the outer side is called the exopodite, and that on the inner side the endopodite. The protopodite consists of two segments, the first very short, and the second much longer. It can easily be seen that the side-plates of the tail-fan (the middle plate, as already mentioned, is the telson) are simply the swimmerets of the sixth abdominal somite. They are much larger than the other swimmerets, and have the endopodite and exopodite broadened out into large plates; while the protopodite is very short, and not divided into segments.

Fig. 4—Walking Legs of Lobster

A, Of first pair; B, of third pair

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If now the third maxilliped ([Fig. 3]) be examined, it will be found that, like the swimmeret, it consists essentially of two branches springing from a stalk of two segments. The exopodite, however, is much smaller than the endopodite, and it ends in a flexible lash made up of many small segments. The endopodite forms the main part of the limb, and has five segments, so that, with the two segments of the protopodite, there are seven segments in the main axis of the limb; the second and third segments are partly soldered together, but the line of union can be plainly seen. Attached to the outer side of the first segment is a membranous plate, known as the epipodite, on which is inserted, near its base, a brush-like structure, which is one of the gills. In the natural position the epipodite and its gill lie in the gill chamber, hidden from view by the side-flap of the carapace.

The legs ([Fig. 4]) can, without difficulty, be seen to consist each of seven segments like those of the maxillipeds, but there is no exopodite. In the young Lobster, when just hatched from the egg, however, each of the legs has a large exopodite like that of the third maxilliped. These exopodites, which are used in swimming, are afterwards lost as the animal grows; but their presence in the young is interesting as confirming the conclusion that the legs, like the maxillipeds, are built on the same plan as the swimmerets. The large claws, and also the first and second pairs of walking legs, end in pincers, or chelæ, the penultimate segment projecting in a thumb-like process against which the last segment works. Each leg, except those of the last pair, has on its first segment an exopodite with a gill like those of the maxilliped.

Fig. 5—Appendages of Lobster in Front of Third Maxilliped

A, Eye-stalk; B, antennule; C, antenna (the flagellum is cut short); D, mandible; E, maxillula; F, maxilla; G, first maxilliped; H, second maxilliped. en, Endopodite; ep, epipodite; ex, exopodite; gn, gnathobases, or jaw-plates; p, palp of mandible; sc, scaphognathite

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Following the series of appendages forwards from the third maxilliped ([Fig. 5]), it is easy to trace the gradual reduction of the endopodite and exopodite; while the two segments of the protopodite become flattened and broadened inwards to form the jaw-plates. The mandibles ([Fig. 5], D), which are the chief organs of mastication, consist mainly of the much enlarged basal segment of the protopodite, with a strongly toothed inner edge, where it works against its fellow of the opposite side; and the rest of the limb is reduced to a small sensory "palp," which represents the second segment of the protopodite and the endopodite.

The antennæ ([Fig. 5], C) can be shown, without difficulty, to conform to the same plan of structure as the other appendages. The two segments of the protopodite are short, but distinct; the endopodite forms the long lash, or flagellum, composed of very numerous small segments; the exopodite is reduced to a small movable scale or spine.

The antennules ([Fig. 5], B) seem at first sight to present the two-branched type of structure in its simplest form; but there is considerable doubt as to whether the two lashes which each bears on a three-segmented stalk are really equivalent to the endopodite and exopodite.

The movable stalks which carry the eyes ([Fig. 5], A) have been considered by some to belong to the series of the appendages, and to be, in fact, modified limbs. If this be the case, we have here the greatest simplification which the limb undergoes in the Lobster, for each eye-stalk consists only of two segments: the first small and incompletely formed, the second in the form of a short cylinder, having the eye at its end. There are, however, reasons for doubting whether the eye-stalks are really appendages.

The hard outer covering of the Lobster not only protects and gives support to the internal organs, but also affords points of attachment for the muscles by means of which the animal moves. In other words, it plays the part of a skeleton; but since, unlike the skeleton of vertebrate animals, it is outside instead of inside the soft parts of the body, it is known as an exoskeleton. Closer examination shows that this outer covering is really continuous over the whole of the body and limbs, but is thin and soft at the joints, allowing the parts to move one upon another. It is composed of a horn-like substance known as chitin, which, except at the joints, is hardened by the deposition in it of carbonate and other salts of lime.

As this external covering does not increase in size after it has been formed, and as it cannot stretch to any great extent, the Lobster requires to cast its shell at intervals as it grows. In this process of moulting the integument of the back splits between the carapace and the first abdominal somite. The body and limbs are gradually worked loose and withdrawn through the opening, leaving the cast shell with all its appendages almost entire. The new covering, which had been formed underneath the old before moulting, 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 salts.

Fig. 6—Dissection of Male Lobster, from the Side. (From British Museum Guide.)

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The internal anatomy ([Fig. 6]) presents many points of interest which can only be briefly touched on here. The food-canal consists of a short gullet leading into a capacious stomach, from which the straight intestine runs to the vent on the under-side of the telson. The stomach has a most remarkable and complicated structure. It consists of two chambers, a larger in front and a smaller behind, which are lined by a continuation of the chitinous outside covering of the body. This chitinous lining is thickened in places to form a system of plates and levers connected with three strong teeth set in the narrow opening between the two chambers. By the action of muscles attached to certain of these plates the teeth work together so as to divide up the food more finely than had been done by the mandibles and other jaws. The whole apparatus, in fact, serves as a kind of gizzard, and is known as the gastric mill.

A small part of the intestine at the hinder end is lined, like the stomach, by a continuation of the chitinous covering, which is turned in at the vent. This lining and that of the stomach, with the plates and teeth of the gastric mill, are cast and renewed when the shell is moulted.

On each side of the food-canal in the thorax lies a large mass of soft tissue, yellowish-green in colour. This is the digestive gland, or "liver," which secretes the digestive juice, discharging it into the food-canal by a short duct on each side just behind the stomach.

The heart lies in the middle of the back, just under the hinder part of the carapace, and gives off, in front and behind, a number of arteries which carry the blood to the various organs of the body. From the smaller branches of these arteries the blood passes, not, as in vertebrate animals, into capillaries, but into the spaces lying between the organs of the body, and it finds its way back to the heart, not in definite veins, but by ill-defined venous channels which open into the pericardium, or space surrounding the heart. From the pericardium the blood enters the heart by six openings in its walls, each guarded by a pair of valves which close when the heart contracts, and prevent the blood from returning to the pericardium.

Fig. 7—Gills of the Lobster, exposed by cutting away the Side-flap of the Carapace (Branchiostegite)

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The venous channels which convey the blood back to the heart are so arranged that most of the blood passes first through the gills, for the purpose of respiration, before it reaches the heart and is again distributed through the body. These gills, as already mentioned, lie in the two branchial chambers under the side-flaps of the carapace ([Fig. 7]), and are attached, some to the epipodites of the thoracic limbs (as described above), and some to the soft membrane of the joints between the limbs and the body; while others are attached to the side-wall of the thorax itself. Each gill is somewhat like a bottle-brush in shape, consisting of a central stalk set round with rows of soft hair-like processes. As the blood streams through the minute channels inside these filaments, it is separated only by a thin membrane from the surrounding water, and the absorption of oxygen and discharge of carbon dioxide can go on easily. For this purpose, however, it is necessary that the water within the gill chamber should be constantly renewed, and this is effected in the following way: the front part of the gill chamber forms a narrow channel running forward under the side-wall of the carapace. Within this channel lies a large plate known as the scaphognathite, attached to the outer side of the maxilla, which during life is constantly in movement, causing a current of water to flow forwards through the channel. The water enters the gill chamber by the narrow slit-like space between the lower edge of the carapace and the bases of the legs, and is discharged in front at the sides of the head, where its movement is helped by the vibrating exopodites of the maxillipeds.

At the sides of the stomach, in the front part of the head, lie a pair of glands which, from their colour, are known as the green glands. These are the excretory organs, corresponding in function to the kidneys of the higher animals. Each has connected with it a thin-walled bladder, which opens to the outside through a small perforation on the under-side of the first segment of the antenna.

The chief part of the nervous system is the ventral nerve-chain, which runs along the under-side of the body. This is a long cord having at intervals a series of knots or swellings, the ganglia or nerve-centres, from which nerves are given off to the appendages and to the organs of the body. In the hinder part of the thorax and in the abdomen there is a ganglion in each somite, but in front these ganglia become crowded together and coalesced, so that we find only a single large ganglion, corresponding to the somites from that of the mandibles to that of the third maxillipeds. Between the ganglia the cord is really double, although for the greater part of its length the two parts are more or less completely fused into one. In front of the head and above the gullet is a ganglion which sends nerves to the eyes, antennules and antennæ, and is known as the brain, although it is, perhaps, hardly so important as that name would suggest. It is connected with the ventral chain by two cords that pass on either side of the gullet.

The eyes, as already mentioned, are set on movable stalks, so that they can be turned in any direction at the will of the animal, and are of the type known as "compound eyes." If the convex black area at the end of the eye-stalk be examined with a strong lens, it will be seen that the membrane which covers it is divided up into a beautifully regular series of square facets. This membrane is a thin and transparent continuation of the chitinous covering of the body, and if it be stripped off and examined under a microscope, it will be found that each facet is capable of acting as a lens and forming an image of external objects. It is not to be supposed, however, that the Lobster sees a separate image in each of the facets, some thirteen thousand in number, which go to make up each eye. In the interior of the eye, at some distance from the surface, are a large number of rod-like bodies, connected with the fibres of the optic nerve, and believed to be the actual organs for the perception of light. Each rod corresponds to one of the facets, and as it lies at the bottom of a long conical tube, of which the walls are covered with dark pigment, it can only receive light from a single point in line with the axis of the tube. In this way the image of any object will be built up, like a mosaic, out of the impressions of light and darkness received through the separate facets, and transmitted to the underlying rods. It has been shown in some Crustacea that, when the animal is in a very dim light, the curtain of pigment separating the tubes is partially withdrawn, so that the light from each facet can reach, not one, but several rods. In this way the images of objects received are much brighter, although they are less sharply defined.

It might be thought that in animals like the Lobster, enclosed in a hard shelly covering, the sense of touch must be very dull, if not altogether absent. This, however, is not the case. What is probably a very delicate tactile sense is provided for by the numerous hairs which are found, of many sorts and sizes, all over the body and limbs. Each of these hairs is really a hollow outgrowth of the chitinous covering, containing a delicate prolongation of the soft tissues underneath, and also supplied, in many if not in all cases, with a nerve-fibre, so that the slightest movement of the hair caused by contact with a solid body is perceived by the animal. Many of these hairs are themselves beset with delicate secondary hairs, arranged so that the whole looks like a feather or like a bottle-brush. These hairs are adapted for detecting slight movements or vibrations in the surrounding water.

Whether Crustacea living in water can hear, in the sense in which the word is used of animals living in air, is doubtful; but it is certain that they are extremely sensitive to vibrations only a little coarser, so to speak, than those we know as sound. The Lobster, and many other Crustacea, do indeed possess a structure which was long supposed to be an organ of hearing, and may possibly in part fulfil that function, although it is now known that that is not its only or even its chief use. It consists of a small cavity in the basal segment of the stalk of the antennule, opening to the outside by a narrow slit on the upper surface of the segment. The cavity is lined by a delicate continuation of the chitinous covering of the body, and has on its inner surface a series of feathered hairs of the kind described above, which are richly supplied with nerve-fibres from a large nerve entering the base of the antennule. Within the cavity, and for the most part entangled among these hairs, are a number of grains of sand. When the Lobster moults, the lining membrane of this cavity is thrown off like the rest of the exoskeleton, and with it the contained sand-grains. While the shell is still soft after moulting, and the lips of the slit are not rigid, as they afterwards become, fresh sand-grains find their way into the cavity to take the place of those which have been cast off. Perhaps, like some other Crustacea, the Lobster buries its head in the sand to insure that some grains may find their way in; for its pincers are too clumsy for it to pick up sand-grains and to place them in the cavity, as some Prawns have been seen to do. At all events, if a freshly moulted Prawn be placed in a vessel of sea-water, and supplied, instead of sand, with powdered glass or metal filings, particles of glass or metal will after a short time be found in its antennular cavities. This habit has been utilized in a very ingenious experiment by which the function of these organs was demonstrated. A Prawn had been induced in this way to place particles of iron filings in the cavities, and a strong electro-magnet was brought near the side of the vessel in which it was kept. It was observed that the Prawn, which had been swimming in the usual horizontal position, at once turned the under-side of its body towards the magnet, and swam about on its side as long as the magnet was in action. When the current exciting the magnet was cut off, the animal resumed its ordinary position. This experiment shows that these organs, to which we may now give their proper name of statocysts, are organs for perceiving the direction of the force of gravity. The magnetic force acted on the particles of iron in the same way that the force of gravity acts on the sand-grains in normal conditions, and the Prawn felt the weight of them, so to speak, pulling towards the side instead of the bottom of the vessel, and turned its body accordingly, to swim, as it supposed, right side up. It is now known that those parts of the human ear called the "semicircular canals" have a somewhat similar function as "organs of orientation," although to animals walking on the solid ground this function is not so important as it doubtless is to animals swimming in water.

The sense of smell is believed to have its seat chiefly in the antennules. The outer branch of each antennule bears tufts of peculiar hairs, in which the chitinous covering is extremely delicate, so that substances dissolved in the water can easily pass through and affect the nerve-endings within. These hairs are known as "olfactory filaments."

The sense of taste in aquatic animals is, perhaps, not sharply defined from that of smell, but it is not very rash to assume that certain hairs on the mouth parts and on the fleshy upper and lower lips which bound the opening of the mouth have to do specially with this sense.

The relative importance of the various senses in the Lobster is well illustrated in the following account of its habits given by Dr. H. C. Williamson in the Report of the Scottish Fishery Board for 1904. After noticing that, in daylight at least, the Lobster appears to be "purblind," only distinguishing light from shadow, Dr. Williamson goes on: "It tests a shadow with its antennæ, or sometimes, when a strong shadow is thrown on it, it jumps at it with its chelæ outstretched and snapping. It is dependent on its antennæ for guiding it in safe places. It is especially careful in testing any hole before it is satisfied with it. It discovers the cavity by means of its antenna, which is waved well out to the side and in front as it walks. It searches the innermost depths of the hole with the antenna, and then inserts its chela. If the examination with the chela is also satisfactory, it immediately turns and backs smartly into the hole. In feeding it is guided to the food by the antennules. A piece of food which is dropped near a Lobster may fall quite unnoticed unless it happens to touch the antenna or the [legs]. It is not seen at all. But sooner or later, according as the distance is short or great, the scent of the food, carried by the currents set up by the exopodites of the maxillipeds, reaches the Lobster. The Lobster is immediately excited, although previously it was lying quite inert in its hole. It whips the water with its antennules in a staccato fashion, and feels about with the antennæ and chelæ, at first without leaving its hole. At once both antennules are seen to be whipping in the direction in which the food is lying, and an active search is made with the antennæ. If they do not succeed in locating the bait, the Lobster rather reluctantly leaves its hole, but cautiously, feeling all round about with its antennæ. It goes off straight in the direction in which the food is lying, and, if it misses it with its antennæ and chelæ, walks over it and gets it with its chelate [walking legs]; it usually picks up its food with the second [walking leg]. Meanwhile the expected feast has by association stimulated the maxillipeds, which are actively working as if they were already masticating the food. Once the food is seized it is conveyed to the maxillipeds, and the Lobster retreats to its hole, there to enjoy its meal."

Lobsters, like most other Crustacea, are of separate sexes. The females (see [Plate I].) may be distinguished from the males by the fact that the abdomen is broader and has deeper side-plates, and by differences in the form of the first two pairs of swimmerets. In the female the first pair, which have only one branch, are short and slender filaments, while in the male they are stout and peculiarly twisted rods. The second pair in the female are similar in form to the succeeding pairs, but in the male they have an additional lobe on the inner branch. The openings of the generative organs will be found in the male on the basal segments of the last pair of legs, while in the female they occupy the same position on the legs of the last pair but two. The testis of the male lies in the thorax, just below the heart. The ovary, which has the same position in the female, is usually much more conspicuous, and from its red colour in the cooked Lobster it is known as the "coral." On the under-side of the thorax of the female, between the last two pairs of legs, is a three-lobed structure enclosing a cavity known as the "sperm-receptacle." Its function is to receive the fertilizing substance from the male, and to retain it until the eggs are ready to be deposited.

PLATE I

MALE AND FEMALE LOBSTERS, SHOWING THE DIFFERENCE IN THE RELATIVE BREADTH OF THE ABDOMEN IN THE TWO SEXES. THIS FIGURE ALSO ILLUSTRATES THE DISSIMILARITY OF THE LARGE CLAWS AND THE FACT THAT THE "CRUSHING-CLAW" MAY BE ON EITHER THE RIGHT OR LEFT SIDE OF THE BODY

(From Brit. Mus. Guide)

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In the Lobster, as in many other Crustacea, the eggs are carried by the female until they hatch. After being extruded from the oviducts, they are attached by a kind of cementing substance to the swimmerets, where they hang in bunches. The swimmerets are kept constantly moving, so that the eggs may obtain the oxygen necessary for the developing embryos within. A female Lobster carrying eggs in this way is said by the fishermen to be "in berry," and may carry, according to its size, from about 3,000 to nearly 100,000 eggs. A period of about ten months elapses between the deposition of the eggs and hatching.

Fig. 8—First Larval Stage of the Common Lobster. × 4. (After Sars.)

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The young Lobster when first hatched ([Fig. 8]) differs considerably in general appearance from the adult animal. The abdominal somites have a row of spines down the middle of the back, and the telson has a forked shape. There are no swimmerets, but, as already mentioned, the legs bear large exopodites, which are used like oars, and by means of these the larval Lobster swims about at the surface of the sea. The claws or chelæ are at first hardly larger than the other legs, but later they increase in size, the swimmerets are developed, the exopodites of the legs are lost, and the young Lobster, sinking to the bottom of the sea, takes on the creeping habits and gradually assumes the shape of the adult.

In many Crustacea the changes of shape or metamorphoses undergone after hatching are much greater than in the Lobster. Some of these changes and their probable significance will be considered at greater length in a later chapter.

The two large claws of the Lobster (see [Plate I].) are not quite alike in size or in shape. The smaller of the two has the inner edges of the fingers sharp and set with saw-like teeth; the larger has the fingers armed with blunt rounded knobs. The larger claw is adapted for crushing the shells of the animals on which the Lobster feeds, while the smaller serves for holding and tearing the prey. In the Lobster, as in many of the higher Crustacea in which this asymmetry occurs, the larger claw may be indifferently on either side of the body. There are certain cases, however, among Crabs where the large claw is constantly on the same side of the body, or, in other words, all the individuals are either right-handed or, more rarely, left-handed.

If a Lobster be caught by one of its claws or by a leg, it very readily parts with the limb in its struggles to escape; and if one of the limbs be crushed or otherwise injured, it is often cast off by the animal. The separation always takes place at the same point, near the base of the limb, and is not simply due to the limb breaking at its weakest part. It is a reflex act, brought about by a spasmodic contraction of some of the leg muscles. At the place of separation, corresponding to the junction of the second and third segments of the limb, which, as already mentioned, are soldered together, the internal cavity is crossed by a transverse partition, having only a small aperture in the centre through which the nerves and bloodvessels pass. When the limb is cast off, this small opening quickly becomes closed by a clot of blood, and further bleeding is stopped. If, as sometimes happens, a limb which has been seriously injured is not cast off, the animal not infrequently bleeds to death. This power of self-mutilation or autotomy, as it is called, is frequently used by Crustacea as a means of escaping from their enemies, and is closely connected with the power of regeneration of lost appendages. Beneath the scar which forms on the stump of a separated limb a sort of bud grows, and gradually assumes the form of the lost segments. At the next moult this straightens out, and, increasing in size at succeeding moults, it ultimately provides, in normal cases, a new member similar in every detail to that which had been lost. Occasionally it happens, under circumstances not yet altogether understood, that the process of regeneration may, so to speak, go wrong, and in this way various malformations and abnormalities result. For instance, it has been found that, if the larger, crushing claw of a very young Lobster be removed by operation or by accident, the limb which grows in its place may assume the form of the smaller, toothed claw. Further, in some other Crustacea (but not in the Lobster, except in the very youngest stages), it is found in such cases that, after removal of the large claw, the claw of the other side assumes at the next moult the form of a crushing claw, so that there is a "reversal of asymmetry."

A still more remarkable change sometimes occurs when one of the eye-stalks is injured. If only the tip of the eye-stalk be cut off, so that the nerve-ganglion which lies in the basal part of the stalk remains uninjured, it will be found that a normal eye is in course of time regenerated. If, however, the whole eye-stalk be amputated, and with it the optic ganglion, there grows in its place, not a new eye-stalk, but a segmented appendage similar to one of the flagella of the antennules. This fact is considered by some zoologists to indicate that the eye-stalks are, like the antennules, true appendages, homologous with the mouth parts and limbs, but this is a much-disputed question into which we cannot enter further here.

Lobsters vary a good deal in colour, but as a rule a living Lobster is of a more or less mottled dark blue, becoming nearly black on the back, and shaded off into orange yellow or red on the under-side. This coloration resides in the shell, and does not change much after the shell has hardened. In this respect the Lobster is unlike many of the smaller Crustacea which have a thin and more or less transparent exoskeleton, and in which the colour resides in certain living cells (chromatophores) of the underlying skin. Many of these Crustacea possess the power of changing their colours to a remarkable degree, by the expansion and contraction of the branched chromatophores.

The question which is often asked, "Why does a Lobster turn red when it is boiled?" is one to which it is not easy to give a simple answer. A chemical change takes place under the influence of heat in the pigment of the shell, which changes it from blue to red; how slight the change is, is perhaps shown by the fact that occasionally living Lobsters are found of a red colour almost as brilliant as that which is assumed on boiling.

Fig. 9—Side-view Of Rostrum of (A) Common Lobster (Homarus gammarus) and (B) American Lobster (Homarus americanus)

The Common Lobster is found on the coasts of Western Europe, from Norway to the Mediterranean, living in shallow water, generally a little way below low-tide mark, wherever a rough, rocky bottom affords suitable lurking-places. On the Atlantic coast of North America, Lobsters are also found abundantly in similar situations. These American Lobsters, if examined carefully, will be found to differ from the European kind in certain small details of structure, of which the most conspicuous is the presence, on the under-side of the rostrum, of two spines or teeth. In the European Lobsters the under-side of the rostrum is smooth ([Fig. 9]). In the nomenclature of technical zoology, these two kinds or species of Lobster are said to constitute (along with a third species found at the Cape of Good Hope) the genus Homarus, the European species being known as Homarus gammarus, and the American as Homarus americanus. The so-called "Norway Lobster" or "Dublin Prawn," which differs from the Common Lobster in having large kidney-shaped eyes and long and slender claws, and in many other details of structure, is placed in a distinct genus, and is known as Nephrops norvegicus. The genera Homarus and Nephrops, together with some others, constitute the family Homaridæ, which again is grouped with other families in a tribe, Nephropsidea, forming a part of the order Decapoda. These groups are intended to express the varying degrees of resemblance and difference in structure between the species of animals which make up the class Crustacea. Since we have good grounds for believing that all these species have arisen by some mode of evolution, this classification also represents the varying degrees of actual relationship between the different forms, so far as this relationship can be discovered. In the next chapter a brief sketch of the chief subdivisions of the Crustacea is given, with such details as to the characteristics of each as are necessary to render intelligible the succeeding chapters on their habits and modes of life.

[CHAPTER III]

THE CLASSIFICATION OF CRUSTACEA

Table of Classification of Crustacea

Fig. 10—The "Fairy Shrimp" (Chirocephalus diaphanus), Male. × 2. (After Baird.)

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Occasionally there may be found in rain-water puddles and the like, in the South of England, a beautiful, transparent, shrimp-like animal, an inch or more in length, to which the name of "Fairy Shrimp" has been given ([Fig. 10]). It is known in technical zoology as Chirocephalus diaphanus, and is a representative of the subclass Branchiopoda. The members of this group are distinguished from other Crustacea by their flattened, leaf-like feet, each of which is divided into a number of lobes, and has a gill plate on the outer side. In Chirocephalus there is no carapace, and the head is followed by eleven distinct body segments, each bearing a pair of leaf-like, or rather fin-like, feet. The hinder part of the body has no appendages, and ends in a forked tail. In the female a large pouch hangs from the under-side of the body, just behind the limb-bearing part, and is often found filled with eggs. In the male, a pair of remarkable-looking appendages, each shaped somewhat like a hand with webbed fingers, hang in front of the head. These are connected with the antennæ, and are known as the "claspers," from their function in seizing and holding the female. The eyes are set on movable stalks. Those Branchiopoda which, like Chirocephalus, have no carapace, form the order Anostraca.

PLATE II

Apus cancriformis, FROM KIRKCUDBRIGHTSHIRE. (SLIGHTLY ENLARGED)

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A second order, the Notostraca, is represented by Apus cancriformis ([Plate II].), which occurs in many places in Europe in ponds and puddles, and very rarely indeed in Britain. In Apus there is a large dorsal shield, or carapace, covering the greater part of the body, which consists of a large number of segments (about twenty-eight), and ends behind in a pair of long antenna-like filaments. The fin-like feet are also very numerous (about sixty-three pairs). The eyes are not stalked, but are set close together on the upper surface of the carapace.

Fig. 11—Estheria obliqua, One of the Conchostraca. (After Sars, from Lankester's "Treatise on Zoology.")

A, Shell of female, from the side; B, male, from the side, after removal of one valve of the shell. (Enlarged.) a′, Antennule; a″, antenna; ad, muscle which draws together the valves of the shell; f, tail fork; md, mandible

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Fig. 12—Daphnia pulex, a Common Species of "Water-flea." Much enlarged. (From British Museum Guide.)

Female carrying eggs in the brood-chamber

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The third order of the Branchiopoda, the Conchostraca ([Fig. 11]), are not represented in Britain, though several species occur on the Continent of Europe. In these the carapace forms a bivalved shell, completely enclosing the body and limbs, and closely resembling that of a small Mollusc.

The fourth order, the Cladocera, comprises the so-called "Water-fleas," which are abundant everywhere in ponds and lakes ([Fig. 12]). They are all of small size, almost or quite microscopic. The carapace, as in the Conchostraca, forms a bivalved shell, but does not enclose the head. There is a single large eye, which really corresponds to two eyes fused together. A pair of large antennæ, each with two branches, carrying long feathered hairs, project at the sides of the head, and are used in swimming with a peculiar jumping motion, from which the popular name of the animals is derived. There are not more than six pairs of feet. The "Water-fleas," of which Daphnia pulex is one of the commonest species, are very beautiful and interesting objects for microscopic examination, on account of their transparency, which allows many details of their internal structure to be studied in the living animal.

The Ostracoda ([Fig. 13]), which form the second subclass in the system of classification here adopted, are nearly all microscopic animals, and are found abundantly in fresh water as well as in the sea. The carapace forms a bivalved shell, which completely encloses the body and limbs, and is often sculptured in an elegant fashion. The Ostracoda are remarkable for the very small number of their appendages. There are not more than two pairs of limbs behind the maxilla. Most of the species are included in two orders, the Myodocopa and the Podocopa, of which the former may generally be distinguished by a notch in the anterior part of the margin of the shell ([Fig. 13], A, n). In the Podocopa the margin is entire.

Fig. 13—Shells of Ostracoda. Much enlarged. (From Lankester's "Treatise on Zoology," after Brady and Norman, and Müller.)

A, Philomedes brenda (Myodocopa); B, Cypris fuscata (Podocopa); C, Cythereis ornata (Podocopa). n, Notch characteristic of the Myodocopa; e, the median eye; a, mark of attachment of the muscle connecting the two valves of the shell. A and C are marine species; B is from fresh water

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Fig. 14—Cyclops albidus, a Species of Copepod found in Fresh Water. (After Schmeil.)

Female specimen carrying a pair of egg-packets. The actual length is about one tenth of an inch

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The subclass Copepoda comprises animals, for the most part of microscopic size, which are abundant in fresh water and in the sea. The common fresh-water genus Cyclops ([Fig. 14]) furnishes a good example of the type of structure characteristic of the class. The body is somewhat pear-shaped, with a narrow abdomen ending in a "caudal fork." The body is divided into somites, and there is no overlapping carapace, although the head and the first two thoracic somites are coalesced. There are four pairs of two-branched, oar-like, swimming feet, and a fifth pair, found in some other Copepoda, is represented in Cyclops by minute vestiges on the first segment of the narrow posterior part of the body. The antennules are very large, unbranched and composed of numerous segments; the antennæ are much smaller. In addition to the usual mandibles, maxillulæ, and maxillæ, there is a pair of maxillipeds which really represent the first pair of trunk limbs. There is a single red eye in the middle of the front of the head. This eye is not formed, like the single eye of the Cladocera, by fusion of a pair of eyes, but it corresponds to a median eye of simple structure which is found in the Branchiopoda, Ostracoda, and many other Crustacea, in addition to the paired compound eyes. From the fact that this median eye is the only one present in the earliest larval stage of Crustacea, the Nauplius (see Chapter IV.), it is sometimes known as the "nauplius eye." The female Cyclops carries her eggs until they hatch, in two oval packets attached to the sides of the body.

Forming a separate order (Branchiura) apart from the more normal Copepoda (order Eucopepoda) is the little group of the Carp-lice, one of which, Argulus foliaceus, is common in England, living as a parasite on different species of fresh-water fish, and often found swimming free in ponds and rivers. It has a broad, flat, and very transparent body, about three-sixteenths of an inch in length. It differs from Cyclops in a great many points, of which, perhaps, the most conspicuous is the possession of a pair of true compound eyes in addition to the median eye. On the under-side of the head are a pair of large round suckers, by means of which the animal fixes itself on to its prey. A study of their development shows that these suckers are really the maxillæ, which in the young animal are jointed limbs ending in a strong claw, but later become changed into the suckers of the adult. A sharp spine, which can be protruded in front of the mouth, is connected with what is believed to be a poison-gland. The eggs are not carried in packets by the female as in Cyclops, but are deposited on stones or water-weeds.

The fourth subclass, Cirripedia, comprises the Barnacles and Acorn-shells. These are very unlike any of the other Crustacea, and, in fact, they were long classed by naturalists with the Mollusca. It was not until their larval development was made known that they were recognized as Crustacea. The common Goose Barnacle (Lepas anatifera—[Plate III].) is found adhering to the bottoms of ships and to floating timber. It has a fleshy stalk or peduncle which is fixed at one end to the supporting object, and bears at the other end a shell, made up of five separate plates, enclosing the body of the animal. The stalk corresponds to the front part of the head, and careful examination may discover at its end, among the hardened cement which fixes it to the support, the remains of the antennules by which the attachment of the young animal was first effected. The body of the animal within the carapace or shell bears the usual mandibles, maxillulæ, and maxillæ, close to the mouth, and six pairs of long, tendril-like feet. These feet have each two branches, composed of numerous short segments and fringed with long hairs. They can be protruded from the slit-like opening of the shell, forming a sort of "casting-net" for the capture of minute floating prey.

The Acorn-shells, of which one species (Balanus balanoides—[Plate III].) is abundant everywhere on our coasts, covering rocks and stones just below high-water mark, differ from Lepas and its allies in having no peduncle. The shell is cemented directly to the rock, and is conical in shape, like a small limpet, with a hole at the top which is closed by four movable valves.

PLATE III

GROUP OF SPECIMENS OF THE GOOSE-BARNACLE (Lepas anatifera), ONE SHOWING THE CIRRI EXTENDED AS IN LIFE. (NATURAL SIZE)

(From Brit. Mus. Guide)

GROUP OF A COMMON SPECIES OF ACORN-SHELL OR ROCK BARNACLE (Balanus Balanoides) (NATURAL SIZE)

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The Stalked Barnacles, like Lepas (suborder Pedunculata), and the Sessile Barnacles, or Acorn-shells, like Balanus (suborder Operculata), together form the order Thoracica. Of the other orders which compose the subclass Cirripedia, the only one that need be mentioned here is the Rhizocephala, which comprises strangely degenerate parasites living on other Crustacea.

The Cirripedia are unlike nearly all other Crustacea in the fact that, with few exceptions, they are hermaphrodite, having both sexes united in each individual. In certain species of the Stalked Barnacles, however, there are minute male individuals that are attached, like parasites, to the large hermaphrodites. In a few species the large individuals only possess female organs, so that the separation of the sexes is complete.

The remarkable larval metamorphoses of Cirripedes and the modifications of structure presented by some parasitic forms will be described in later chapters.

The fifth and last subclass, that of the Malacostraca, is by far the largest and most important, and will require to be considered in more detail than any of the others. The animals composing the various orders into which the subclass is divided differ very greatly in structure, but they all agree in having typically the same number of appendages as the Lobster—namely, nineteen pairs (or twenty, if the eye-stalks be included). They also agree in the very important character that the trunk limbs are divided into two sets, thoracic and abdominal, the former of eight, and the latter of six pairs.

Fig. 15—Nebalia bipes. Enlarged. (From Lankester's "Treatise on Zoology," after Claus.)

a′, Antennule; a″, antenna; ab¹-ab⁶, the abdominal limbs; ad, muscle joining the two valves of the shell; f, tail-fork; p, palp of maxillula; r, rostral plate; t, telson; 1-7, the seven somites of the abdomen

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The first order of the Malacostraca, the Nebaliacea, comprises a few Crustacea of small size, which differ in some very important characters from all the other orders. Nebalia bipes ([Fig. 15]), which occurs on the southern coasts of the British Isles, has a large bivalved carapace enclosing most of the limbs. In front, a small "rostral plate" is joined to the carapace by a movable hinge, and partly covers the stalked eyes. The eight pairs of thoracic feet are all alike, and are flattened and leaf-like in form, resembling those of the Branchiopoda. The first four pairs of abdominal limbs are large two-branched swimming feet, but the last two pairs are reduced to small vestiges. Two of the most important points in which the Nebaliacea differ from all the other Malacostraca are that there are seven instead of six somites in the abdomen (the last somite has no appendages), and that the telson has connected with it a pair of movable rods forming a "caudal fork" like that of the Branchiopoda. On account of the leaf-like thoracic feet and the possession of a caudal fork and other features, the Nebaliacea were formerly classified with the Branchiopoda, but a closer examination of their structure has shown that they are true Malacostraca. In having an additional somite in the abdomen and in other points, however, they may be regarded as forming a link between the Malacostraca and the lower forms of Crustacea, and for this reason they are set apart as a series Leptostraca, while the other orders form a series Eumalacostraca.

The orders of the Eumalacostraca, again, are grouped, as shown in the table of classification, into four divisions. The first of these, the Syncarida, includes only one order, comprising a few small Crustacea (see [Fig. 84], p. 264) which have recently been discovered in fresh water in Tasmania and Australia. They have no carapace, and all the thoracic somites, or all but the first, are distinct. The antennules are two-branched, the antennæ may have a scale-like exopodite, and the last pair of abdominal appendages form, with the telson, a tail-fan. The eyes are sometimes stalked, but in one species they are sessile. The thoracic limbs, which are not clearly divided into maxillipeds and legs, carry a double series of plate-like gills or epipodites. As will be shown later, the living Syncarida are especially interesting on account of their resemblance to certain very ancient fossil Crustacea.

The second division of the Eumalacostraca, the Peracarida, includes five orders, the members of which differ very greatly in appearance. They all agree, however, in certain important points of structure, of which the most conspicuous is the possession, in the female sex, of a brood-pouch for carrying the eggs and young. This brood-pouch is formed by a series of overlapping plates attached to the bases of the thoracic limbs.

Fig. 16—Mysis relicta, One of the Mysidacea. Enlarged. (From Lankester's "Treatise on Zoology," after Sars.)

cs, Cervical groove of the carapace; m, brood-pouch

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The first order of the Peracarida, the Mysidacea, consists of small, free-swimming, shrimp-like animals ([Fig. 16]). Many species are common in the sea round the British coasts, and from their possession of a brood-pouch, in which the young are carried, they are sometimes known as "Opossum Shrimps." The eyes are stalked, and the carapace is well developed, although it does not unite with all the thoracic somites. The antennæ have a flattened, scale-like exopodite, probably of use for keeping the animal balanced in swimming. Only one pair of the thoracic limbs are modified to form maxillipeds, and all the legs (as in the larval Lobster) have exopodites which form the chief swimming organs. The uropods and telson form a "tail-fan." One of the most curious points in the organization of some Mysidacea is the possession of a pair of statocysts in the endopodites of the uropods. Each statocyst consists of a small cavity containing a cake-shaped concretion known as a "statolith," resting on a group of sensory hairs. There is reason to believe that these organs have the same function as the statocysts of the Lobster, although they are placed at the other end of the body. The statolith serves the same purpose as the sand-grains found in the Lobster's statocyst, although, unlike these, it is not introduced from outside, but is formed in position by secretion from the walls of the sac.

Fig. 17—Gnathophausia willemoesii, One of the Deep-sea Mysidacea. Half Natural Size. (From Lankester's "Treatise on Zoology," after Sars.)

gr, A groove dividing the last abdominal somite

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Most of the Mysidacea have no special organs of respiration, that function being discharged (as in many of the smaller Crustacea) by the general surface of the body, and especially by the thin carapace; but certain deep-sea Mysidacea ([Fig. 17]) have tufted gills attached at the base of the thoracic legs. In all cases the maxilliped has a plate-like epipodite, which lies under the side-fold of the carapace, and no doubt assists respiration, causing by its movements a current of water to flow under the carapace.

Fig. 18—Diastylis goodsiri, One of the Cumacea. Enlarged. (From Lankester's "Treatise on Zoology," after Sars.)

a′, Antennule; l¹-l⁵, the five pairs of walking legs; m, brood-pouch; ps, "pseudo-rostrum" formed by lateral plates of the carapace; t, telson; ur, uropods

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The members of the second order of the Peracarida, the Cumacea ([Fig. 18]), are small marine Crustacea in which the anterior part of the body is generally stout, while the abdomen is slender and very mobile. The short carapace does not cover more than the first three or four of the thoracic somites. The eyes are not stalked, and are usually fused together to form a single organ on the front part of the head. Swimming branches (exopodites) are usually present on some of the thoracic legs, at least in the males, which are more active swimmers than the females. In the males, also, the swimmerets of the abdomen are often more or less developed, but they are always absent in the females. The uropods do not form a tail-fan, but are slender forked rods carrying comb-like rows of spines, said to be used in cleaning the anterior appendages from the mud among which these animals generally live. The telson is often absent, or, rather, it is coalesced with the last somite of the abdomen. Under the side-fold of the carapace on each side lies, as in the Mysidacea, the epipodite of the maxilliped; but in this order it forms a gill, and usually carries a row of flattened gill lobes.

Fig. 19—Apseudes spinosus, One of the Tanaidacea. Enlarged. (From Lankester's "Treatise on Zoology," after Sars.)

ex, Vestiges of exopodites on second and third thoracic limbs; oc, the small and immovable eye-stalks; sc, scale or exopodite of antenna; ur, uropod

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Fig. 20—A Woodlouse (Porcellio scaber), One of the Isopoda. Enlarged. (From Lankester's "Treatise on Zoology," after Sars.)

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The third Order, that of the Tanaidacea ([Fig. 19]), is of special interest, since in many respects it forms a transition to the next. It comprises a number of minute Crustacea, generally found burrowing in mud in the sea. They have a small carapace, which only involves the first two thoracic somites, the rest of the somites being distinct. The side-folds of the carapace enclose a pair of small cavities, within which lie, as in the case of the last two orders, the epipodites of the maxillipeds. The eyes are not movable, although they are set on little side-lobes of the head, representing the vestiges of eye-stalks. The first pair of thoracic limbs are maxillipeds, and the second pair are very large, and form pincer-claws (chelæ). Minute vestiges of exopodites are sometimes found on the second and third pairs, but they are not used for swimming, and only help to keep a current of water flowing through the gill cavities. The abdomen is very short, with small swimmerets, and the telson is not separated from the last somite. The uropods are generally very small, and do not form a tail-fan.

Unlike the Tanaidacea, the Isopoda, which form the fourth order of the Peracarida, are very numerous in species, and very varied in structure and habits. The most familiar are the Woodlice, or Slaters, which are commonly found in damp places, under stones and the like. Besides these, however, the order includes a vast number of forms living in the sea and a few that live in fresh water. The examination of a common Woodlouse, such as Oniscus or Porcellio ([Fig. 20]), will give a general idea of the form and structure of a typical Isopod, although many curious modifications are found, some of which will be mentioned in later chapters.

There is no distinct carapace, but the last vestige of one may be indicated by the fact that the first thoracic somite is completely fused with the head. All the other somites of the body are distinct (in some Isopods, however, the abdominal somites are coalesced), but the telson is not separate from the last somite. The eyes are not stalked, but are sessile on the sides of the head. The antennules have only a single branch, and in the Woodlice are very small. The antennæ have no exopodite, although in a few other Isopods a minute vestige is present. The thoracic limbs never have any trace of exopodites. The first pair are maxillipeds, and if they carry an epipodite it is never enclosed in a gill cavity, as in Tanaidacea. The swimmerets form one of the most characteristic features of the Isopoda, for they are always flattened into thin plates, which act as gills. In the Woodlice, which breathe air, certain curious modifications of the swimmerets are found, which will be described in a later chapter. In some Isopods that live as parasites on fish or on other Crustacea, each individual is at first a male, and later becomes a female. They are almost the only Crustacea, except the Cirripedes already mentioned, which are normally hermaphrodite.

Fig. 21—An Amphipod (Gammarus locusta). Enlarged. (From Lankester's "Treatise on Zoology," after Sars.)

a′, Antennule; a″, antenna; acc, accessory (inner) flagellum of antennule; br, gill plate; cx, coxal plate (the expanded first segment of the leg); gn, the two pairs of gnathopods (prehensile legs); plp′′′, abdominal appendage of third pair; prp′, prp″, first and second peræopods, or walking-legs; t, telson; ur, uropod; II, VIII, second and eighth thoracic somites; 1, 6, first and sixth abdominal somites

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The fifth order of the Peracarida, the Amphipoda, is also a very large one. The "Sand-hoppers," which are very common on sandy coasts, belong to this order, as do also a very large number of other forms found in the sea and in fresh water, which have no popular names. A common species is Gammarus pulex, sometimes called the "Fresh-water Shrimp," which is found everywhere in streams and ditches. Several closely allied species, such as G. locusta ([Fig. 21]), are found in the sea. The body is flattened from side to side, and the abdomen is generally bent upon itself. There is no carapace, but, as in the Isopods, the first thoracic somite is fused with the head. The eyes are sessile on the sides of the head. The antennules have a small inner branch, and the antenna have no exopodites. The thoracic limbs, of which the first pair form maxillipeds, have no exopodites, and are partly hidden by a row of shield-like plates along the sides of the thorax. These plates are formed by the enlarged and flattened basal segments of the limbs themselves, and on the inner side they carry a series of oval plates, which are the gills. The abdominal appendages are divided into two sets: the first three pairs have each two slender, many-jointed branches, and are used in swimming; the last three pairs are short, stiff, and directed backwards, and are used in pushing the animal through mud or among water-weeds. In many Amphipods, such as the Sand-hoppers, the last three pairs of abdominal limbs are used in jumping by sudden backward strokes of the abdomen.

Fig. 22—Two Species of Caprellidæ. (From Lankester's "Treatise on Zoology," after Sars.)

A, Phtisica marina, a species which retains the fourth and fifth pairs of thoracic limbs (prp′, prp″); B, Caprella linearis, in which these limbs are represented only by the gills (br). (Enlarged.) a′, Antennule; a″, antenna; abd, vestigial abdomen; gn, gnathopods; m, brood-pouch; IV, V, fourth and fifth thoracic somites

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Fig. 23—Paracyamus boopis, the Whale-louse of the Humpback Whale. (From Lankester's "Treatise on Zoology," after Sars.)

A, Male, dorsal view, enlarged; B, the maxillipeds detached and further enlarged. a′, Antennule; a″, antenna; abd, vestigial abdomen; br, gills; gn, gnathopods; IV, V, fourth and fifth thoracic somites

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Two families of the Amphipoda differ so much in general appearance from the others that they deserve mention. The Caprellidæ ([Fig. 22]) have the body drawn out to a thread-like slenderness, and the abdomen reduced to a mere vestige. The fourth and fifth pairs of thoracic limbs are generally absent, though the corresponding gills remain. The animals live in the sea, clambering among sea-weeds or zoophytes in a fashion which recalls the movements of "looper" caterpillars. The Cyamidæ, or "Whale-lice" ([Fig. 23]), are, as the name implies, parasites on the skin of whales, and are closely related to the Caprellidæ. They have, however, a broad, flattened body, more like that of an Isopod than an ordinary Amphipod, and their legs have strong curved claws with which they cling to the skin of their host.

The third division of the Malacostraca, the Eucarida, consists of two orders of very unequal interest and importance. The first of these, the order Euphausiacea ([Fig. 24]), comprises only a single family of small, shrimp-like Crustacea found swimming freely at the surface or in the depths of the sea. In these the carapace fuses with all the thoracic somites, the eyes are stalked, the antennules have two flagella, and the antennæ have a broad scale. None of the thoracic limbs are modified into maxillipeds, and all carry swimming exopodites. The uropods and telson form a tail-fan. A single series of feathery gills are attached to the bases of the thoracic limbs. Nearly all the Euphausiacea possess the power of emitting light, and are furnished for the purpose with a number of organs which were formerly supposed to be "accessory eyes."

Fig. 24—Meganyctiphanes norvegica, One of the Euphausiacea. Twice Natural Size. (From Lankester's "Treatise on Zoology.")

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The second order of the Eucarida, the Decapoda, is by far the largest of the orders of Crustacea, and it includes all the larger and more familiar members of the class. It is necessary, therefore, to give a considerably fuller account of its subdivisions than has been given in the case of the other orders. The typical characters of the Decapoda are well illustrated by the Lobster, which has been already described. As in the Euphausiacea, the eyes are stalked, and the carapace fuses with all the thoracic somites. From the Euphausiacea the Decapoda differ in the fact that three pairs of the thoracic limbs are modified as maxillipeds, the remaining five pairs forming the "ten legs" to which the name of the order alludes. Further, the gills are arranged in more than one series, not all attached to the bases of the legs, as in the Euphausiacea, and covered over by the side-flaps of the carapace instead of being freely exposed. While agreeing in these essential characters, however, the members of the order Decapoda differ very widely among themselves in structure and in general form, and they are classified (in the arrangement adopted here) in two suborders, which are again subdivided into sections and tribes.

The suborder Natantia includes the numerous species of what are commonly known as Prawns and Shrimps. These are characteristically powerful swimmers, with lightly armoured bodies, more or less flattened from side to side, with a thin, saw-edged rostrum, and with large swimmerets which are the chief organs of swimming; in addition, some of the more primitive Natantia have swimming branches, or exopodites, like those of the Euphausiacea, on the thoracic legs. This suborder is divided into three tribes. The Penæidea include the large Prawns of tropical seas (Penæus—[Plate IV].), which have the first three pairs of legs provided with chelæ, and not differing greatly in size. The Stenopidea are a small group of forms resembling the Penæidea in having chelæ on the first three pairs of legs, but the third pair are much larger than the others. The Caridea comprise our common Prawns (Leander, Pandalus) and Shrimps (Crangon), besides a host of less generally known forms; in these the third legs are never chelate, although the first and second often are.

PLATE IV

Penæus caramote, FROM THE MEDITERRANEAN. (ABOUT HALF NATURAL SIZE) (From Brit. Mus. Guide)

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The second suborder, that of the Reptantia, is much more diversified, but the animals composing it are united by certain characteristics, of which the most obvious are their creeping habits (although some species can swim well), their heavily armoured bodies, often more or less flattened from above downwards, with the rostrum never thin and saw-edged, and the swimmerets not used to any great extent for swimming.

The first section of the Reptantia, the Palinura, includes the Spiny Lobsters, Rock Lobsters, or Sea-Crawfish, and their allies, forming the tribe Scyllaridea. They are distinguished by having no large pincer-claws, though the last pair of legs may have small pincers in the female sex. One species, the Common Spiny Lobster ([Plate V].), is found on the southern and western coasts of the British Islands. The other tribe belonging to this section is the Eryonidea, comprising a number of small lobster-like forms living in the deep sea. They have pincer-claws on the first four, or on all five, pairs of legs, and they are of special interest on account of their geological antiquity.

PLATE V

THE COMMON SPINY LOBSTER (Palinurus vulgaris). (MUCH REDUCED) (From Brit. Mus. Guide)

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The section Astacura contains only a single tribe, Nephropsidea, formed by the true Lobsters and the fresh-water Crayfishes. They have pincer-claws on the first three pairs of legs, and the first pair are much larger than the others.

The third section of the Reptantia, the Anomura, comprises forms in which the abdomen is variously modified, being either bent upon itself or, if extended, more or less soft and feebly armoured. The last pair of legs are commonly reduced in size, and not used in walking. The members of the four tribes composing the section differ widely in their general appearance.

The Galatheidea ([Plate VI].) are small, flattened, lobster-like animals which have the abdomen bent under the body. In one family (Porcellanidæ) the animals have quite the appearance of little Crabs (see [Fig. 41], p. 113), but they may be distinguished from the true Crabs (Brachyura) by the fact that there are only three pairs of walking legs behind the great chelæ, the last pair of legs being very small and carried folded up at the sides of the body, or even within the gill chambers.

PLATE VI

Munida rugosa. BRITISH. (REDUCED)

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The Thalassinidea are small lobster-like animals which burrow in sand and mud, and have generally a more or less soft abdomen (see [Fig. 38], p. 103).

PLATE VII

THE COMMON HERMIT-CRAB, Eupagurus bernhardus, IN THE SHELL OF A WHELK (REDUCED)

(From Brit. Mus. Guide)

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The tribe Paguridea includes the Hermit Crabs (Paguridæ) and their allies. The typical Hermit Crabs ([Plate VII].), which are familiar objects in seaside rock-pools, live in the empty shells of Whelks and other Gasteropod Molluscs, which they carry about with them as portable shelters. The structure of the animals is modified in adaptation to this curious habit. The abdomen, which is protected during life by the borrowed shell, is soft and unarmoured, and is spirally twisted. The swimmerets, which have only the function of carrying the eggs in the female, are much reduced, and are usually present only on one side of the body. The uropods no longer form a tail-fan, but are adapted for firmly wedging the hind part of the body into the coils of the shell. One of the chelipeds is much larger than the other, and serves to block up the opening when the animal withdraws into its shelter. In tropical countries certain Hermit Crabs (Cœnobitidæ) have become adapted to a life on land, and one of these, the well-known Coconut Crab, or Robber Crab (Birgus latro), which is the largest species of the tribe, has given up the habit of protecting itself with a shell, and its abdomen has again acquired a strong armour on the upper side. The marine Lithodidæ—to which the British Stone Crab, Lithodes maia ([Plate VIII].) belongs—seem at first sight to have little resemblance to the Hermit Crabs, for they have the abdomen very small, and tucked up under the body as in the true Crabs. Like the Porcellanidæ, mentioned above, however, the Lithodidæ have only three pairs of walking legs behind the chelipeds, the last pair being feeble and usually folded out of sight within the gill chambers. The relationship of the Lithodidæ to the Hermit Crabs is shown by the abdomen, which is more or less twisted to one side, and has swimmerets only on one side in the female, and quite wanting in the male.

PLATE VIII

THE "NORTHERN STONE-CRAB," Lithodes maia, MUCH REDUCED. THE LAST PAIR OF LEGS ARE FOLDED OUT OF SIGHT IN THE GILL CHAMBERS

(From Brit. Mus. Guide)

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The Hippidea are curious little Crabs found burrowing in sandy beaches in the warmer seas. They have the abdomen tucked under the body, and the legs flattened for shovelling the sand.

The Brachyura, or true Crabs, form the fourth section of the Reptantia, and are distinguished by having the abdomen reduced to a tail-flap, which is doubled up under the cephalothorax, and is usually without any trace of the uropods which are present in all the groups already mentioned, with the single exception of the Lithodidæ. At the sides of the head the side-plates of the carapace become firmly soldered to the "epistome," a plate which lies in front of the mouth, and in this way there is formed the "mouth-frame," within which lie the jaws, covered in by a pair of "folding-doors" formed by the flattened third maxillipeds.

PLATE IX

THE COMMON SHORE-CRAB (Carcinus mænas). (REDUCED)

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Dromia vulgaris, CARRYING ON ITS BACK A MASS OF THE SPONGE Clione celata. BRITISH. (REDUCED)

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The first tribe of the Brachyura, the Dromiacea, comprises a number of Crabs that in many points of structure resemble the Lobsters, and are regarded as the most primitive members of the section. Dromia vulgaris ([Plate IX].), a furry, clumsy-looking Crab, occasionally found on our southern coasts, has the last two pairs of legs short and carried up over the back, where they are used for holding a mass of living sponge which the Crab uses as a cloak to protect and conceal itself. At the sides of the abdomen, wedged in between the telson and the last somite, a pair of small plates may be seen, which are the last vestiges of the uropods. These are wanting in the other tribes of the Brachyura.

PLATE X

Calappa flammea, BRAZIL. (REDUCED)