Читать книгу The Life of Crustacea - W. T. Calman - Страница 9
ОглавлениеFig. 1—The Common Lobster (Homarus gammarus,) Female, from the Side. (From British Museum Guide.)
View larger image
Fig. 2—One of the Abdominal Somites of the Lobster, with its Appendages, separated and viewed from in Front. (From British Museum Guide.)
View larger image
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.)
View larger image
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
View larger image
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
View larger image
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.)
View larger image
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)
View larger image
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)
View larger image
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.)
View larger image
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.
