PLATE XIIIPortunus depuratorA SWIMMING CRAB,Portunus depurator.BRITISH.(REDUCED)View larger imageMaia squinadoA SPIDER-CRAB,Maia squinado,DRESSED IN FRAGMENTS OF WEED.BRITISH(REDUCED)View larger image
PLATE XIII
Portunus depuratorA SWIMMING CRAB,Portunus depurator.BRITISH.(REDUCED)View larger image
A SWIMMING CRAB,Portunus depurator.BRITISH.(REDUCED)
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Maia squinadoA SPIDER-CRAB,Maia squinado,DRESSED IN FRAGMENTS OF WEED.BRITISH(REDUCED)View larger image
A SPIDER-CRAB,Maia squinado,DRESSED IN FRAGMENTS OF WEED.BRITISH(REDUCED)
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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 ofHyas(H. araneusandH. coarctatus) andMaia(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 offthe 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 CrabStenorhynchus(orMacropodia)longirostrispicking 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—seeFig. 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 assemblageof 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. Inthe 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.
PLATE XIVPortunus depuratorCorystes cassivelaunus.MALE (ON LEFT) AND FEMALE (ON RIGHT). BRITISH (REDUCED)View larger imageMaia squinadoAlbunea symnista,ONE OF THE HIPPIDEA. INDIAN SEAS. (REDUCED)View larger image
PLATE XIVPortunus depuratorCorystes cassivelaunus.MALE (ON LEFT) AND FEMALE (ON RIGHT). BRITISH (REDUCED)View larger image
PLATE XIV
Corystes cassivelaunus.MALE (ON LEFT) AND FEMALE (ON RIGHT). BRITISH (REDUCED)
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Maia squinadoAlbunea symnista,ONE OF THE HIPPIDEA. INDIAN SEAS. (REDUCED)View larger image
Albunea symnista,ONE OF THE HIPPIDEA. INDIAN SEAS. (REDUCED)
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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 thistube communicates at its base with the passages through which the water passesoutfrom 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 drawndownthe 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.
InAlbunea(Plate XIV.), which belongs to this tribe, a long "antennal tube," which looks very like that ofCorystes, 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 inCorystes, 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 ofRemipesin 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.
Callianassa stebbingiFig. 38—Callianassa stebbingi(Female),a Sand-burrowing Thalassinid from the South Coast of England.Natural SizeView larger image
Fig. 38—Callianassa stebbingi(Female),a Sand-burrowing Thalassinid from the South Coast of England.Natural Size
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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 generaOcypodeandGelasimusand some of their allies.
PLATE XVOcypode cursorOcypode cursor.WEST AFRICA. (REDUCED)View larger imageGelasimus tangeriGelasimus tangeri.MALE ABOVE, FEMALE BELOW. WEST AFRICA. (REDUCED)View larger image
PLATE XV
Ocypode cursorOcypode cursor.WEST AFRICA. (REDUCED)View larger image
Ocypode cursor.WEST AFRICA. (REDUCED)
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Gelasimus tangeriGelasimus tangeri.MALE ABOVE, FEMALE BELOW. WEST AFRICA. (REDUCED)View larger image
Gelasimus tangeri.MALE ABOVE, FEMALE BELOW. WEST AFRICA. (REDUCED)
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Some of the species ofOcypode(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 arepractically 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 genusGelasimus("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.
InOcypodeandGelasimusthe 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 exclusivelyterrestrial 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 ofGrapsusand 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 isTalitrus saltator(Fig. 39), butOrchestia gammarellusis 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 abovehigh-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.
The Common Sand-hopperFig. 39—The Common Sand-hopper(Talitrus saltator),Male, from the Side.× 3. (After Sars.)View larger image
Fig. 39—The Common Sand-hopper(Talitrus saltator),Male, from the Side.× 3. (After Sars.)
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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" (seeFig. 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.
Piece of a Tropical Sea-weedFig. 40—A,a Piece of a Tropical Sea-weed(Halimeda); B,a Crab(Huenia proteus)which lives among the Fronds ofHalimeda,and closely resembles them in Form and Colour. Reduced.(After Borradaile.)
Fig. 40—A,a Piece of a Tropical Sea-weed(Halimeda); B,a Crab(Huenia proteus)which lives among the Fronds ofHalimeda,and closely resembles them in Form and Colour. Reduced.(After Borradaile.)
The little Crabs of the family Leucosiidæ (Oxystomata), of which the British representatives are several species of the genusEbalia, 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 ofHuenia 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-weedHalimeda, 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 aschromatophores, 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 contractinto 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 genusGelasimus, 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 uniformlycoloured 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 assumedat 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.
The Common Porcelain CrabFig. 41—The Common Porcelain Crab(Porcellana longicornis),slightly enlarged, and One of the Third Maxillipeds detached and further enlarged to show the Fringe of Long HairsView larger image
Fig. 41—The Common Porcelain Crab(Porcellana longicornis),slightly enlarged, and One of the Third Maxillipeds detached and further enlarged to show the Fringe of Long Hairs
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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 shorecollector will often find, on turning over a large stone, one or more specimens of the little Porcelain Crabs (Porcellana platycheles, orP. 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 isBalanus 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 everyday; 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 theBroad-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."
It has already been mentioned that the animals living on the sea-bottom in shallow water do not differ greatly in character from those that may be found between tide-marks. As we go farther out from land, however, into the deeper water, the character of the fauna gradually changes. One by one the species found near the shore become rare and disappear, and their places are taken by others characteristic of the intermediate depths. These in their turn give way to others, till in the abysses of the great oceans we find an assemblage of strange animals adapted to the conditions of life in the great depths, and differing widely in many respects from the more familiar inhabitants of the coastal waters. In this "fauna of the deep sea," which extends to the greatest depths reached by the dredge or trawl, the Crustacea occupy a prominent place. Before proceeding to discuss some of these peculiar forms, however, it is necessary to attempt to form some idea of the conditions under which they live.
In the first place, the character of the sea-bottom changes very greatly as we pass away from the coast. Near the shore it is extremely diversified, consisting in one place of rocks swept bare by the tides or overgrown with jungles of sea-weed, in another of banks of gravel or shingle, of sand or of mud, but in all cases derived from the "waste" of the land, as it is eaten away by the waves or washed down by the rivers. As the distance from land increases, the deposits become finer and finer, till they shade off into a soft oozy mud, composed of the finest particles brought down by the rivers. In the neighbourhood of large rivers this mud may sometimes extend for hundreds of miles from the land, but there is a limit to the distance to which even the finest particles can drift before they settle to the bottom, and beyond this limit the floor of the ocean is covered by sediments which owe their origin, not to the land, but to the ocean itself. The surface waters of the ocean everywhere teem with a vast variety of floating animals and plants, and, as these die, their remains sink to the bottom "like a perpetual shower of rain."
Among the most abundant floating organisms—in the warmer seas, at any rate—are certain minute animals known asForaminifera, which belong to the lowest class of the animal kingdom, and have shells composed, in most cases, of carbonate of lime. Over vast areas the bottom of the oceanis covered with a soft grey ooze, made up almost entirely of the dead shells of Foraminifera rained down from above. Since the commonest species of Foraminifera found under these circumstances belong to the genusGlobigerina, the deposit is known as "Globigerina ooze."
In certain regions of the ocean the shells of other floating organisms largely replace those of the Foraminifera in covering the ocean floor, and in the deepest abysses—so deep that the shells of surface animals are dissolved before they can sink to the bottom—there is found a deposit known as the "red clay," which appears to be derived largely from the impalpable volcanic and cosmic dust that floats in the atmosphere. It is not necessary for our present purpose to enter more fully into the interesting questions connected with these deep-sea deposits, but it is important to remember that, generally speaking, the floor of the deep sea is everywhere soft ooze, without rocks or stones, except for an occasional water-logged lump of pumice or a stone dropped by a melting iceberg. This fact is probably of great importance in the life of deep-sea animals.
One of the most peculiar and characteristic of the physical conditions in the deep sea is the enormous pressure under which life has to be carried on. At the surface of the sea the pressure of the atmosphere is, roughly speaking, 14½ pounds per square inch.At a depth of only 33 feet of water this pressure is doubled, and at greater depths the pressure increases in proportion, till at 2,000 fathoms it is more than 2½ tons on the square inch. As a matter of fact, however, the animals at the bottom of the sea are probably but little affected by this enormous pressure. Only, when they are brought up by the dredge the sudden release of pressure causes the fluids of the body to expand and destroys the tissues, so that the animals are generally dead or dying when they reach the surface.
More important than the pressure in its influence on life is the darkness of the depths. The light of the sun only penetrates the water of the sea to a comparatively small depth. At 200 fathoms there is not enough light to produce any effect on a photographic plate. Even at a considerably less depth the absence of light puts an end to all plant-life, except for the ubiquitous bacteria, and it follows that all the animals of the deep sea ultimately depend for their food-supply on the rain of dead bodies of surface animals which, as already mentioned, is constantly falling on the sea-bottom.
The temperature at the bottom of the deep sea is always very low. Dr. Alcock states that "in the open part of the Bay of Bengal, where the mean surface temperature is about 80° F., the temperature at a depth of 100 fathoms is only about 60° F., at a depth of 300 fathoms not quite 50° F.; while at adepth of 2,000 fathoms the temperature all the year round is only 3° above freezing-point."
Finally, it is important to notice theuniformityof the conditions at the bottom of the sea; not only are the alternation of night and day and the progress of the seasons unfelt in the abysses, but the conditions are practically the same over vast areas in all the oceans.
In the case of deep-sea Crustacea, we are frequently confronted with a difficulty which does not occur in the case of some other groups of animals—Corals or Echinoderms, for example—the difficulty, namely, of deciding whether the animals really lived on or near the bottom, or were captured by the open mouth of the trawl on its way to the surface. When the animals are plainly not well adapted for swimming—as, for instance, most of the Crabs—it may be assumed that they did actually live on the bottom; but, with the prawn-like forms, the possibility that they may really be inhabitants of the intermediate depths must always be taken into consideration.
A Deep-sea LobsterFig. 42—A Deep-sea Lobster(Nephropsis stewartii),from the Bay of Bengal. Reduced.(After Alcock and Anderson.)View larger image
Fig. 42—A Deep-sea Lobster(Nephropsis stewartii),from the Bay of Bengal. Reduced.(After Alcock and Anderson.)
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Munidopsis regiFig. 43—Munidopsis regia,a Deep-sea Galatheid from the Bay of Bengal.Reduced.(After Alcock and Anderson.)View larger image
Fig. 43—Munidopsis regia,a Deep-sea Galatheid from the Bay of Bengal.Reduced.(After Alcock and Anderson.)
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In animals that live in perpetual darkness we should expect to find, in accordance with the principle of adaptation which runs through the whole of organic nature, that the eyes are wanting or imperfectly developed. In a great many deep-sea animals this is indeed the case. The deep-sea Lobsters of the genusNephropsis(Fig. 42), whichare very closely allied to the Norway Lobster (Nephrops) of shallow water, have very short and slender eye-stalks hidden under the rostrum, and showing at the tip only the merest traces of what was once an eye. In the lobster-like Eryonidea (seeFig. 46, p. 133), the reduced eye-stalks are firmly fixed in notches in the front edge of the carapace. Some of the deep-sea Crabs and Prawns seem also to be totally blind. In a great many cases degenerationhas not quite gone so far, and the eyes are present, although much reduced and modified. Thus, the very numerous deep-sea species of Galatheidæ, belonging to the genusMunidopsis(Fig. 43) and itsallies have, as Alcock says, "pallid, milky-yellow, lack-lustre eyes which, though they may perhaps serve to distinguish between light and darkness, can never form a definite visual image." It is probable, indeed, that these pale-coloured eyes are specially adapted for vision in a dim light, for it has been shown that in certain deep-sea Euphausiacea the pigment-sheaths between the separate elements of the compound eyes are greatly reduced, and are fixed in the position temporarily assumed by those in the eyes of normal Crustacea when kept in the dark. Be this as it may, there are many deep-sea Crustacea which have well-developed and darkly-pigmented eyes. Some of these are swimming forms, which may at times migrate into the upper strata of water to which some rays of light penetrate; but there are some cases of Crabs and other bottom-living species that have well-developed eyes, although they live at great depths. This would seem to suggest that, although shut off from the light of day, they are not condemned to grope in perpetual darkness. Many deep-sea animals are known to be phosphorescent, and it seems probable that the large-eyed species may profit by the light emitted by the glow-worms and fireflies of the abyss. Thus, Alcock points out that the deep-sea Hermit CrabParapagurus pilosimanus(Plate XVI.), which lives in partnership with a colony of Sea-anemones which it carries about with it, has large eyes, although itdescends to depths of at least 2,000 fathoms; and he suggests that the Crab may be able to see its way by the light emitted by the zoophytes.
Deep-Sea Hermit CrabPLATE XVIA DEEP-SEA HERMIT-CRAB,Parapagurus pilosimanus,SHELTERED BY A COLONY OFEpizoanthus.FROM DEEP WATER OFF THE WEST OF IRELAND(SLIGHTLY REDUCED)View larger image
PLATE XVI
A DEEP-SEA HERMIT-CRAB,Parapagurus pilosimanus,SHELTERED BY A COLONY OFEpizoanthus.FROM DEEP WATER OFF THE WEST OF IRELAND(SLIGHTLY REDUCED)
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Some of the Crustacea, however, are themselves luminous. Thus, Alcock records how specimens of a deep-sea Prawn,Heterocarpus alphonsi, "poured out, apparently from the orifices of the 'green glands' at the base of the antennæ, copious clouds of a ghostly blue light of sufficient intensity to illuminate a bucket of sea-water so that all its contents were visible in the clearest detail." Certain other Prawns are known to possess special light-producing organs on various parts of the body and limbs. It is in the Euphausiacea, however, that these organs have been most fully examined, and although the members of this group (seeFig. 24, p. 56) are by no means all deep-sea animals, some of them occurring at the surface of the sea, the structure of their luminous organs, or "photophores," may appropriately be described here. They are situated on the under-surface of the abdomen, in the basal segments of some of the thoracic legs, and on the upper surface of the eye-stalks. Each consists of a globular capsule covered by a layer of pigment, except on the outer side, where there is a transparent biconvex lens. In the centre of the capsule is a peculiar "striated body" which seems to be the actual seat of luminescence, and behind it is a concave reflector composed of concentric lamellæ, and having a silvery lustre. Beforetheir luminosity was observed, these organs were described as "accessory eyes," but there can be little doubt that they serve rather as searchlights, although, from the positions that some of them occupy on the body, it is not easy to see how they can illuminate objects within range of the eyes. That the function of phosphorescent organs is not always that of giving light for their possessor to see by is shown by the fact that many luminous animals are blind. It is important to notice, however, that these blind animals never have complex "photophores" like those just described, but only exhibit a diffuse luminosity or give off luminous secretions; as an example among Crustacea, the blind Eryonidea (seeFig. 46, p. 133) may be mentioned, one species of which was observed by Alcock to be "luminous at two points between the last pair of thoracic legs, where there is a triangular glandular patch." In a recent discussion of the whole question of phosphorescence in marine organisms, Dr. Doflein concludes that the part it plays in the life of the animal probably differs in the different cases. In some it may serve to attract prey, as moths are attracted to a candle; in others it may help individuals of the same species to keep together in a swarm or to find their mates, the varying arrangement of the photophores producing characteristic light-patterns that serve as "recognition marks" like the colour-patterns of animals that live in the light of day. The clouds of luminous secretion thrownout byHeterocarpusand other Prawns, and by certain Mysidacea and Ostracods, may serve to baffle pursuers, like the cloud of ink thrown out by a Cuttlefish, and in some cases the more complex organs may illuminate objects within the range of vision. That this does not exhaust the possibilities of speculation on the subject, however, is shown by the case of certain deep-sea Prawns which have been recently discovered to possess photophores placed so as to illuminate the interior of the gill cavities. What function they can discharge in this position seems beyond conjecture.
The colours of deep-sea Crustacea are very curious. Few of them have the blanched appearance common, for instance, in animals that live in the darkness of caves; on the contrary, their colours are often very vivid, but they are nearly always uniform, without spots or markings, and in a large proportion of cases are in some shade of red or orange. This red colour seems to be associated, in some way that we do not understand, with the darkness of their habitat. The general absence of markings is very striking. Dr. Alcock remarks that in deep-sea Crustacea we never see "those freaks of colour, or those labyrinthine mottlings and dapplings, that excite our curiosity when handling the Crabs and Shrimps of the reefs." Possibly the explanation of this may be that in these dwellers in darkness colour is merely, as it were, an accident, aby-product of physiological processes directed to other ends, not a character of protective or warning value, as in animals that hunt and are hunted in the light of day. It is a curious fact, which may have some bearing on this problem, that in many cases, while the adults are coloured in some shade of red, the eggs carried by the female are bright blue or green.
Some of the peculiarities of structure observed in deep-sea Crustacea seem to be correlated with the difficulties of resting or moving about with security on the soft ooze of the sea-floor. Among the Crabs we find a preponderance of long-legged species, not only among the true Spider Crabs (Oxyrhyncha), but also in other groups (Dromiacea likeLatreillia, figured onPlate XIX., and Oxystomata), the members of which assume the same spider-like form. In some cases the legs are fringed with long stiff hairs, which may help to prevent the animal from sinking in the ooze, and the spines on the body and legs of many species may have the same effect. Among the deep-sea Prawns, the species of the family Nematocarcinidæ (Plate XVII.) have extremely long and slender legs, which we may assume to be used like stilts for walking over the soft ooze.