CHAPTER V

CRUSTACEA OF THE SEASHORE

The tract of seashore which is laid bare by the retreat of the tide offers on most coasts a rich collecting-ground to the student of Crustacea. In places where shelving, weed-covered rocks run out to sea, innumerable Crustacea have their home in the rock-pools, or lurk in crannies awaiting the return of the tide. On sandy beaches, at first sight apparently barren of life, a closer search will reveal a whole fauna, amongst which burrowing Crustacea of various orders are prominent. Further, the shore collector will find from time to time stray specimens of forms that have their proper habitat beyond low-tide mark, and occasionally their remains are thrown in quantities on the beach by storms. It is convenient, therefore, to treat the Crustacea of the shore as a sample of those inhabiting the shallower waters of the ocean. In these shallower waters--down to the limit where light no longer penetrates from above, where vegetable life ceases, and where the strangely modified inhabitants of the deep sea begin to appear--the sea-bottom is perhaps the most densely populated of all parts of the earth's surface. Nowhere, at all events, do we find so wide a range of animal forms, from the simplest organisms (Protozoa) up to highly-organized Vertebrates. Nowhere, perhaps, is the struggle for existence more keen, and it is not without justice that some naturalists have regarded the shallow waters of the sea as "one of the great battle-fields of life," where, in the long course of evolution, the main branches of the animal kingdom have had their origin.

Conspicuous among the animals of this region are Crustacea of all sorts and sizes. To identify all the species that may be obtained in a single haul of the dredge in British seas would sometimes be a hard task even for the most expert student of the group. Our present purpose, however, is not to compile a faunistic catalogue, but merely to give some idea of the endless diversity of form, and to note a few of the "shifts for a living"--of the ways in which structure and habit are adapted to the conditions of life in the Crustacea of the shore and of shallow water.

Though it might seem that the heavily armoured Lobsters and the larger Crabs would be sufficiently protected against most enemies when once they have attained their full size, yet they are preyed upon by the Octopus, which seizes them with its suckers and pierces their armour with its powerful beak, injecting a poison that paralyzes its victims. Some years ago a "plague" of Octopus very seriously affected the Lobster fishery in the English Channel. To escape from enemies such as these, the Lobsters and many Crabs have the habit of lurking in crevices of the rocks, while in case of sudden alarm the Lobster may escape from danger by swimming, or rather darting, with great swiftness, tail foremost, through the water by powerful strokes of the abdomen and tail-fan. In the more lightly armed Prawns and other Crustacea of the tribe Natantia, which are characteristically swimmers, the power of rapid motion is probably the chief means of protection against enemies. There is reason to believe that the Lobsters have been derived from prawn-like swimming forms which have sacrificed some of their agility in developing their heavy armour-plating, retaining, however, the power of sudden and rapid motion in emergency. This power, again, has been lost by the typical Crabs (Brachyura), in which the abdomen is reduced in size and without a tail-fan, so as to be useless for swimming. While most of the Crabs, however, are somewhat slow of movement, trusting to their armour and their powerful pincers for defence, the Swimming Crabs (Portunidæ--Plate XIII.) have reacquired the power of swimming by means of the paddle-shaped legs of the last pair. Some of the tropical species of Portunidæ are probably the most expert swimmers among the Crustacea, and are described as shooting through the water like fish.

The Lobster's habit of seeking shelter in rock-crevices or under stones is one which is shared by a very large number of shore Crustacea. From some primitive kind of Lobster which discovered the advantages of a portable shelter have been derived the Hermit Crabs. In rock-pools one may often see whelk or periwinkle shells tumbling about with an activity quite foreign to the nature of their original molluscan inhabitants, and closer examination will show that each contains a Hermit Crab, which retreats into the shell when disturbed. If extracted from the shell, the Crab (Fig. 36) can be seen to be most beautifully adapted to its peculiar mode of life. The abdomen is soft and spirally twisted to fit into the interior of the spiral shell, and the uropods, instead of forming a tail-fan, are modified into holding organs, with roughened, file-like surfaces which can be pressed outwards against the walls of the shell, and wedge the body so firmly that an attempt to drag the animal forcibly from its retreat often results in tearing it in half. The front part of the body, which is exposed when the animal is walking, retains its shelly armour. One of the pincer-claws, most commonly the right, is much larger than the other, and serves to block the opening of the shell when the body is withdrawn into it. The next two pairs of legs are long and slender, and are used for walking; but the last two pairs are short, with a roughened surface at the end, and serve to steady the body in the mouth of the shell. The swimmerets on the right side of the body, which is pressed against the central pillar of the shell, have disappeared, but those of the left side remain.

As the Hermit grows, it is necessary for him to remove from time to time into a larger dwelling. It has been stated that he will sometimes dispossess the rightful owner of a whelk-shell for this purpose, dragging him out piecemeal and eating him; but other observers deny that this ever happens, and in most cases, at all events, the Hermit is content to wait until he finds an empty shell of suitable size. After turning this over and exploring the interior with his claws, to satisfy himself that it is unoccupied, he deftly whips the unprotected hinder part of his body into the new habitation, keeping hold of the old one meanwhile, so that he can return to it if the other proves unsuitable. The Hermits are very pugnacious, and fight with one another for the possession of desirable shells, the victor dragging his opponent out and establishing himself in his place. Besides appropriating the shell of a dead Mollusc, many Hermits seem to go into partnership with living animals of various kinds, and some of these associations will be noticed in a later chapter. A number of species adopt other dwellings than molluscan shells, and some tropical Hermits, for instance, are found living in the cavities of water-logged stems of bamboo (Fig. 37); while others, relinquishing the advantages of a portable shelter, live in holes in corals or in the canals of living sponges. Although in some of these cases the body is straight, it usually shows traces of its original adaptation to a spiral shell in having no swimmerets on the left side.

The only Hermits which have a full series of swimmerets are the primitive Pylochelidæ (Fig. 37), which come very near to what we imagine the ancestral form of the group to have been like, and can hardly be separated from the mud-burrowing, lobster-like Thalassinidea. A few Hermits have given up altogether the use of any protective covering. One of these is the Coconut Crab (_Birgus_), to be mentioned when we come to deal with the Crustacea of the land. Another is the Stone Crab (_Lithodes_--Plate VIII.) of our own seas, and its kindred, which have redeveloped shelly plates on the back of the abdomen, but carry it doubled up under the body like the true Crabs. These also preserve some traces of the original twisting of the abdomen, and have swimmerets only on one side.

Some Crustacea construct habitations for themselves. On turning over a flat stone between tide-marks, one often finds a little mass of bits of weed and rubbish attached to it, and if this be torn open a greenish-brown, shrimp-like animal, about three-quarters of an inch long, is seen slithering away on its side. This is an Amphipod (_Amphithoë rubricata_) which builds the shelter for itself, sticking the fragments together with threads of a cementing material produced by glands on the surface of its body and legs. Other Amphipods construct more neatly finished tubular dwellings of mud, or even of small stones, which are attached to sea-weeds and the like; and some make portable shelters of the same kind, which they carry about with them like the caddis-worms of fresh-water streams.

Some of the true Crabs also employ portable shields for purposes of defence or of concealment. The species of _Dorippe_ which are found in tropical seas have the last two pairs of legs short, elevated on the back so that they cannot be used for walking, and ending in a kind of grasping claw. By means of these claws the Crab holds over its back some object, generally one valve of a molluscan shell, sometimes even a mangrove-leaf, to supplement the protection afforded by its carapace. The "Sponge Crabs" (_Dromiidæ_), of which one species, _Dromia vulgaris_ (Plate IX.), occurs on the southern coasts of Britain, have also the last two pairs of legs elevated on the back and used in a similar way; but in this case the covering is usually a mass of living sponge, one of the Sea-squirts (Tunicata), or some similar organism.

Even more remarkable are the "masking" habits of the Spider Crabs (Oxyrhyncha). In these the carapace is almost always covered with sea-weeds, zoophytes, and other organisms which afford a very effective disguise. For example, specimens of the British species of _Hyas_ (_H. araneus_ and _H. coarctatus_) and _Maia_ (_M. squinado_--Plate XIII.), which are very common on our coasts, readily escape the notice of the collector, as they lurk in the rock-pools. They are slow-moving animals, and the carapace and limbs are usually quite hidden by dense tufts of growing sea-weed, sponges, and other organisms. By observing the Crabs in an aquarium, it has been found that they actually dress themselves, plucking pieces of weed and the like and placing them on the carapace, where they are held in position by numerous hooked hairs. The transplanted fragments continue to live and grow until the Crab appears like a miniature moving forest. Still more strange is the fact that the Crabs appear to be able in some degree to adapt the nature of their covering to their surroundings. It has been found that specimens dressed in sea-weeds, when placed in an aquarium among sponges, picked off the weeds from their bodies and limbs, and planted fragments of sponge in their place. Not only does this habit afford the Crabs protective concealment, but it may also in some cases serve as a source of food-supply. The late Dr. David Robertson, of Cumbrae, one of the most observant of marine naturalists, saw the Crab _Stenorhynchus_ (or _Macropodia_) _longirostris_ picking food-particles from among the vegetation on its body, and conveying them to its mouth.

Many Crustacea of different orders seek concealment and protection by burying themselves in sand. A pool left by the tide on a sandy beach may at first sight appear empty of all life, but if it be watched for a little while a greyish, shadowy form may often be seen to dart across it, to settle on the bottom with a little puff of sand, and to disappear. Even a close scrutiny of the spot will hardly discover anything, but with a hand-net one may succeed in scooping up, before it can dart away again, a specimen of the Common Shrimp (_Crangon vulgaris_--see Fig. 78, p. 244), whose translucent body is finely mottled with greyish-brown so as to match exactly the sand among which it rests.

If a spadeful of sand from between tide-marks be stirred up in a bucket of sea-water and allowed to settle for a few seconds, and the water then poured off through a fine muslin net, a wonderful assemblage of minute Crustacea may often be obtained. Numerous species of Ostracods, Copepods, and Amphipods, and some Isopods, can be collected in this way, and some of these, at least, show peculiarities of structure which appear to be adapted to a sand-burrowing habit. Perhaps the most remarkable Crustacea living in such situations, however, are the Cumacea. In these, as already mentioned, the gills, which are attached to the first pair of thoracic limbs, lie one on each side of the thorax in a cavity enclosed by the carapace. These cavities are continued forwards to the front of the head, where they unite in a single opening from which a transparent tube (or a pair of tubes) can be protruded. It appears probable that this very peculiar arrangement of the respiratory system is adapted to enable the animals to breathe while buried in sand or mud. The water is probably drawn in behind through the narrow slit between the side-plates of the carapace and the bases of the legs, and is expelled through the tube which is protruded from the front of the head. In this way the delicate gills are protected from injury and kept from becoming clogged with sand, while the effete water, loaded with the products of respiration, is carried off to a safe distance, so that it does not re-enter the gill chamber.

In the case of such minute forms, however, it is very difficult to determine the precise details of their mode of life by observation of the living animals. In the larger Decapods, which can be watched in their natural haunts, or more closely in aquaria, many interesting adaptations to burrowing in sand have been discovered. Many Crabs belonging to the tribe Brachyrhyncha often take refuge in sand or gravel, burying themselves till only the eyes remain exposed. The Swimming Crabs (Portunidæ--Plate XIII.) of our own coasts have been found to use the paddle-shaped last pair of legs for digging as well as for swimming. In the sand, the Crab keeps its large claws, or chelipeds, folded close up to the front edge of the carapace, which is cut into sharp, saw-like teeth. Between these teeth the water passes, to reach the entrance to the gill chamber which lies at the base of each cheliped, and in this way an efficient strainer is provided, which in coarse sand at least prevents the clogging of the respiratory passages. The out-going current of water from the gills passes through channels that open on either side of the mouth-frame.

A more complex adaptation of structure to the habit of sand-burrowing is found in the Masked Crab (_Corystes cassivelaunus_--Plate XIV.). This Crab is common on the British coast, living in moderately deep water wherever the bottom is sandy, and it has received its English name from the fact that the furrows on the back of the carapace give it a grotesque resemblance to a human face. It is noteworthy, among other things, for the marked difference between the sexes, the male having very long, slender chelipeds, while those of the female are quite short. The most remarkable features of its organization, however, have to do with its habit of burrowing in sand. The antennæ, which in most Crabs are extremely short, are in this species as long as the body, and each bears a double fringe of stiff hairs disposed along the upper and under sides of the antenna, but curved inwards, so that when the two antennæ are brought together parallel with each other, the hairs interlock and form a long tube. At its base this tube communicates with a space in front of the mouth, into which open the channels from the gill chamber at the front corners of the mouth-frame. The Crab burrows in fine sand, and the process is thus described by Professor Garstang: "The Crab sits upright on the surface of the sand; the elongated, talon-like claws of the four hindmost pairs of legs dig deeply into the sand; the body of the Crab is thus forcibly pulled downwards by the grip of the legs, and the displaced sand is forced upwards on the ventral side of the body by the successive diggings and scoopings of the legs; the slender chelate arms of the first thoracic pair assist in the process of excavation by thrusting outwards the sand which accumulates round the buccal region of the descending Crab." In this way the Crab descends deeper and deeper, until nothing is visible above the surface of the sand but the tips of the antennæ. The antennal tube keeps open a channel leading from the buried Crab to the water above. Since this tube communicates at its base with the passages through which the water passes _out_ from the gill chamber in most Crabs, it was assumed by the older observers that the antennal tube served to carry the outflowing water to the surface of the sand. It has recently been shown, however, by Professor Garstang that when the Masked Crab is buried in sand the normal respiratory current is reversed, water being drawn _down_ the antennal tube, into the gill chambers, and passing out through the openings at the base of the chelipeds which, when the Crab is not buried, serve for its entrance.

Most, if not all, of the Crabs belonging to the tribe Oxystomata are sand-burrowers, and the structure of the mouth parts characteristic of the tribe appears to have been acquired as an adaptation to this habit. As already mentioned, the mouth-frame in these Crabs is triangular instead of square, being produced forwards between the eyes, and the third maxillipeds, which cover it, are also elongated. In this way the exhalent channels carrying the water from the gill chambers open on the front margin of the head, and are exposed even when the Crab is buried. In the different families of this tribe the inhalent openings by which the water enters the gill cavities are protected in various ways, and so arranged that respiration can go on without danger of the gills becoming clogged by sediment.

The members of the tribe Hippidea (sometimes called "Mole Crabs"), among the Anomura, have habits somewhat similar to those of the Crabs just described. They are common on sandy beaches in the warmer parts of the globe, and they burrow with great rapidity by means of the curved, flattened end-segments of the legs. The carapace is generally smooth and oval, and the body is compact, the short abdomen being folded up as in the Crabs.

In _Albunea_ (Plate XIV.), which belongs to this tribe, a long "antennal tube," which looks very like that of _Corystes_, is believed to have a similar function in connection with respiration when the animal is buried. In this case, however, the tube is formed, not by the antennæ, as in _Corystes_, but by the antennules, so that it affords a striking example of the independent evolution of similar structures from quite different origins.

_Hippa emerita_, which is found on the coasts of North and South America, has the mouth parts imperfectly formed, and not adapted for biting; and it is stated by Professor S. I. Smith that the animal feeds in the way that an earthworm does, swallowing the sand through which it burrows, and extracting the nutriment which it may contain. This habit, however, is not followed by other members of the tribe, for Mr. Borradaile found that a species of _Remipes_ in the Maldive Islands could "easily be caught by a bait of Crab at the end of a line, pouncing on it with its sharp maxillipeds, and allowing itself to be flicked out of the sand if the rod be sharply lifted."

In the cases mentioned above, the Crustacea do not bury themselves much below the surface of the sand, and do not form definite burrows; but there are many Crustacea which live in open tunnels dug deep into the sand. Some of these belong to the category of amphibious forms, to be mentioned presently; but there are others which live in deeper water, and of which the habits are less open to observation.

Nearly all the Thalassinidea (Fig. 38) live in burrows, often of considerable depth, in sand or mud. Although now classed with the Anomura, these animals are lobster-like in form, loosely built, generally with short carapace and long, soft abdomen. They have usually very small eyes, which appear as if they were not of much use for vision; and some of the hinder pairs of legs are short, and carried folded against the sides of the body, probably for use when the animal is moving up or down in its burrow.

Most of the Stomatopoda resemble the Thalassinidea in their mode of life, and show some curious similarities to them in structure, although by no means closely related. They are described as lying in wait for prey at the mouth of their burrows, darting out on passing fish or other animals, which they seize with their great saw-toothed claws, and retreating with great rapidity to the bottom of the burrow.

Most of the Crustacea mentioned live below tide-marks, and at all events are rarely seen when the sand in which they burrow is left bare by the tide; but there are others, especially on tropical shores, which seem to have their chief period of activity when the sand or mud banks on which they live are exposed to the air. Chief among these amphibious forms in the warmer seas are the Crabs of the genera _Ocypode_ and _Gelasimus_ and some of their allies.

Some of the species of _Ocypode_ (Plate XV.) dig their burrows between tide-marks, where they are swamped by the advancing tide, and must be excavated afresh when the water retreats. Other species, however, live above high-water mark, and are practically terrestrial animals, only entering the water occasionally, and, indeed, unable to survive prolonged immersion. The work of excavating the burrows has been watched in several species. The Crab comes out of the burrow sideways, carrying a load of sand between two of the walking legs on the rear side. By a sudden movement the sand is jerked away to some distance, where it accumulates in a little heap, and the Crab dives into the burrow for another load. Most of the Crabs belonging to this genus possess a curious "stridulating organ" on one of the large claws, by means of which they can produce a buzzing or hissing sound. On the inner surface of the "hand" there is a raised patch, which, when examined with a lens, is seen to be made up of a series of fine ridges, like the teeth of a file. When the limb is bent in towards the body, this patch can be rubbed up and down against a sharp-edged ridge or scraper on the third segment of the limb, and in this way the sound is produced. What the use of the sound may be is not quite clear, but there is probability in Dr. Alcock's suggestion that it serves to warn intruders that the burrow is already occupied. These Crabs run very swiftly, and one species was seen by Professor S. I. Smith to catch Sand-hoppers (Amphipods of the family Talitridæ) by springing on them suddenly, "very much as a cat catches mice," but it also fed on dead fish and the like.

Of somewhat similar habits are the numerous species of the genus _Gelasimus_ ("Fiddler Crabs"--Plate XV.), which abound on sand and mud flats of tropical shores. These little Crabs are remarkable for the great dissimilarity between the sexes in the form of the chelipeds. In the female both chelipeds are small and feeble, but in the males one of them, either the right or the left, is enormously enlarged, sometimes exceeding in length and breadth the body of the Crab which carries it. What the precise use of this enormous claw may be does not seem to be quite certainly known. It is said to be used as a weapon by the males in fighting with one another, but it seems too clumsy to be very efficient for this purpose. It is often brilliantly coloured, and has been supposed to be a sexual adornment.

In _Ocypode_ and _Gelasimus_ the respiratory apparatus is modified for the purpose of breathing air. The gills are similar to those of purely aquatic Crabs, and no doubt serve for respiration when the animal is in the water; but the gill chambers are much more spacious than usual, and the lining membrane is richly supplied with bloodvessels. Air is admitted to the gill chambers by an opening, protected by a brush of hairs, between the second and third pairs of walking legs on each side. It is believed that in this way the gill chamber is fitted to be used as a lung when the animals are out of the water. Similar arrangements in some of the more exclusively terrestrial Crustacea will be mentioned in a later chapter.

There are many Shore Crabs, however, which lead a more or less amphibious existence without showing any marked modifications of structure as compared with their more purely aquatic relatives. On our own coasts, the Common Shore Crab (_Carcinus mænas_--Plate IX.) commonly spends several hours each day exposed to the air, and in an aquarium it will voluntarily leave the water if the opportunity be afforded it. On tropical coasts the species of _Grapsus_ and allied genera are often seen clambering with great agility about exposed rocks.

Analogous habits to those of the sand-burrowing, amphibious Crabs described above are shown on a small scale by the Amphipods of the family Talitridæ, known as "Sand-hoppers" or "Beach-fleas." Everyone who has walked over the firm sand near high-water mark on our own shores must have noticed the myriads of actively hopping little creatures disturbed at every step. The commonest species of Sand-hopper on the British coasts is _Talitrus saltator_ (Fig. 39), but _Orchestia gammarellus_ is also common. Both species occur together on sandy beaches or among decaying sea-weeds, and are among the most important scavengers of the seashore, picking clean the bones of fish or other animals cast up by the tide. In this country the Sand-hoppers do not, as a rule, venture far above high-water mark; but in warmer climates species of Talitridæ live in the damp forests at great distances from the sea, and deserve to be ranked among the terrestrial Crustacea.

It has been mentioned above that the Common Shrimp is protected, not only by its habit of lying half buried in sand, but also by its close resemblance in colour to the sand among which it lives. There are many others among the shore Crustacea which show what seems to be a "protective resemblance" in colour and form to their surroundings. It is necessary to be cautious in interpreting these resemblances as necessarily protective, since the fish and other enemies which prey on these Crustacea see them with eyes very different from ours, and probably, in many cases, are guided to their prey by the sense of smell rather than by sight. The "masking" habit of the Spider Crabs, already described, strongly suggests, however, that concealment from sight is an important protection to some shore Crustacea, and helps to make it probable that the same end is reached in other cases by modifications of form and colour.

There can be no doubt, at all events, that many Crustacea are very inconspicuous to human eyes when they remain motionless in their natural surroundings. Thus, for example, the Caprellidæ, or "Skeleton Shrimps" (see Fig. 22, p. 54), are hard to detect without very close search, as they cling to the feathery branches of the hydroid zoophytes among which they are usually found. They are strangely modified Amphipods, in which the body is slender and thread-like, and generally of a semi-transparent, whitish or yellowish colour, like the zoophytes on which they live. They clamber about among the branches with a movement like that of a "looper" caterpillar, and often remain clinging by means of the hooked claws of the hinder pairs of legs, with the fore part of the body gently waving about.

The little Crabs of the family Leucosiidæ (Oxystomata), of which the British representatives are several species of the genus _Ebalia_, are often extremely like pebbles of the gravel among which they live. In many tropical species the carapace is pitted and eroded, so as to resemble a worn fragment of coral shingle. One of the most striking cases among the Crabs, however, is that of _Huenia proteus_ (Fig. 40), one of the Spider Crabs (Oxyrhyncha), which is found in the Indian and Pacific Oceans. In this little Crab the carapace is flat, and is extraordinarily variable in form. In most of the males it is triangular in outline, but in most of the females and in some males it is broadened by leaf-like expansions of the side edges. Borradaile has pointed out that these broad individuals are usually found among the sea-weed _Halimeda_, and that they closely resemble the fronds of this weed in form and in their greenish colour.

A number of Crustacea are known to possess a chameleon-like power of changing their colour. The mechanism by which this change is effected is similar to that found in other animals, such as fish and frogs, which have the same power. The pigment which gives its colour to the animal is lodged in microscopic star-shaped bodies known as _chromatophores_, lying for the most part just below the skin. Each chromatophore consists of a central body from which a number of branching filaments radiate. The pigment may contract into the centre of the chromatophore, forming a minute and hardly visible speck, or it may spread out into the branching filaments, forming a distinct spot of colour. Each chromatophore may in some cases contain several colours of pigment, and these may expand or contract independently of each other, so that a whole series of changes may be produced by a single chromatophore. In the larger Crabs and Lobsters the visible colour of the animals depends on pigment in the shelly exoskeleton, which is thick enough to hide the chromatophores in the living tissues underneath, and no very rapid or considerable changes are apparent; but in the smaller forms, in which the exoskeleton is thin and translucent enough to allow the underlying colours to appear through it, the changes in the chromatophores may produce striking effects. Thus, Fritz Müller describes a species of Fiddler Crab of the genus _Gelasimus_, in which the hinder part of the carapace was brilliantly white, but five minutes after the Crab was captured it had changed to a dull grey. Many other cases of colour change have been described, but most remarkable and the most fully studied is that of the Prawn, _Hippolyte varians_, which is very common on our own coasts, and has recently been the subject of a very elaborate series of researches by Professors Keeble and Gamble. The specimens of this Prawn show "a bewildering variety of colour and of colour-pattern"; they may be uniformly coloured in various shades of brown, green, or red, or they may be "blotched," "barred," or "lined," with colour. These different varieties are generally found among sea-weeds, which they resemble in colour and pattern, the "lined" forms, for instance, frequenting finely branched and feathery weed. Like many other protectively coloured animals, they are of sedentary habits, clinging to the weed, and seldom moving by day. If a specimen be removed from its habitat and placed in an aquarium with different kinds of sea-weed, it will take refuge among that which it most closely resembles. It appears that this resemblance in colour-pattern is acquired during the growth of the Prawn, and that a young specimen kept among finely branched sea-weed will acquire the "lined" pattern, while others, living among coarser weed, become "barred," "blotched," or "monochrome." Even in the adult Prawns the colour (though not the pattern) becomes changed in a day or two if they are placed among weed of a different colour--from green to brown, or the like. Within certain limits still more rapid changes of colour take place. If kept in the dark, or if placed on a white background (for example, in a porcelain dish) in the light, the Prawn quickly becomes nearly colourless, by contraction of the chromatophores, a transparent bluish tint alone remaining, due to a substance which diffuses from the chromatophores into the fluids of the body. In natural conditions this phase is assumed at night; and the interesting observation has been made that Prawns kept in the dark continue for three or four days to show a periodic expansion and contraction of the chromatophores, corresponding to the alternation of day and night. It seems that the rhythm of light and darkness has become impressed on the chromatophore system of the animal, and the movement of the pigments is regulated by something analogous to memory.

It has already been mentioned, in dealing with the Lobster, that certain Crustacea have the power of voluntarily throwing off some of their limbs (autotomy). In many cases, as in the Lobster, this power is mainly of use in enabling the animal to discard an injured limb; but there are some Crustacea which seem to adopt it as a means of escaping from the attack of an enemy. On our own coasts the shore collector will often find, on turning over a large stone, one or more specimens of the little Porcelain Crabs (_Porcellana platycheles_, or _P. longicornis_--Fig. 41) clinging to its under-side. If these Crabs be seized by one of the large claws, they frequently leave the claw in the captor's hand and scuttle off without it; and it cannot be doubted that, as in the case of lizards and other animals which have a similar power of self-mutilation, this habit often enables them to escape from their natural enemies.

Although the Crustacea as a whole are predominantly active animals, many examples have already been mentioned of species which are more or less sluggish and sedentary in their habits. The extreme degree of passivity is reached by the Barnacles (Cirripedia), which differ from all other Crustacea (except some parasites) in being fixed to one spot, and quite without the power of locomotion in the adult state. Most of the Barnacles met with on the shore or in shallow water belong to the division of the Sessile Barnacles or Acorn-shells (Operculata). Every visitor to the seashore has noticed the little conical shells which cover exposed rocks as if with a coat of rough-cast. On the British coasts the commonest species is _Balanus balanoides_ (Plate III.), though other species closely resembling it are also common. They are to be found almost up to high-water mark in situations where they are left uncovered for many hours every day; but the valves which close the opening of the shell fit so tightly that a little sea-water is enclosed, and the animal is protected from drying up even when exposed to the heat of the sun. If a stone or a chip of rock, with a few of these animals on it, be placed in a jar of sea-water, their peculiar mode of obtaining food can easily be watched. The valves will presently be seen to open a little, and the curled cirri will be protruded, opened out like the fingers of a hand, and withdrawn again with a sort of grasping motion. These movements are continued without stopping while the animal is under water. If the cirri be examined with a pocket-lens or under a microscope, it will be seen that they are fringed with stiff bristles, so that, when they are opened out, the whole forms a kind of "casting-net." As it is swept through the water, this net entangles minute floating particles of animal or vegetable matter, and carries them into the shell, so that they can be seized by the jaws and swallowed. The cirri, as we have already seen, are really the modified thoracic limbs, so that, in Huxley's words, "A Barnacle may be said to be a Crustacean fixed by its head, and kicking the food into its mouth with its legs."

A mode of obtaining food by "net-fishing," not unlike that employed by the Barnacles, is found in certain Crustacea belonging to a widely different group--the little "Porcelain Crabs" (Fig. 41) mentioned above. Mr. Gosse observed that the Broad-clawed Porcelain Crab (_Porcellana platycheles_) employed its third pair of maxillipeds, which are thickly fringed with long feathered hairs, in making alternate casting movements "exactly in the manner of the fringed hand of a Barnacle, of which both the organ and the action strongly reminded me."