Part 6 (1/2)

Some of the true Crabs also employ portable shi+elds 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 soenerally one valve of a rove-leaf, to supplee Crabs” (_Droaris_ (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 si is usually a e, one of the Sea-squirts (Tunicata), or soanism

[Illustration: _PLATE XIII_

A SWIMMING CRAB, _Portunus depurator_ BRITISH (REDUCED)

A SPIDER-CRAB, _Maia squinado_, DRESSED IN FRAGMENTS OF WEED BRITISH (REDUCED)]

Even ” habits of the Spider Crabs (Oxyrhyncha) In these the carapace is alanisuise 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- animals, and the carapace and li sea-weed, sponges, and other organis the Crabs in an aquarium, it has been found that they actually dress the them on the carapace, where they are held in position by nurow until the Crab appears like a e is the fact that the Crabs appear to be able in so to their surroundings It has been found that speci sponges, picked off the weeds froe 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 fro them to its mouth

Many Crustacea of different orders seek conceal themselves in sand A pool left by the tide on a sandy beach ht appear ereyish, 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 ain, a speci 78, p 244), whose translucent body is finely reyish-brown so as towhich 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 e 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 thein such situations, however, are the cuills, 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 h 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 froills are protected froed 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 er Decapods, which can be watched in their natural haunts, oradaptations to burrowing in sand have been discovered Many Crabs belonging to the tribe Brachyrhyncha often take refuge in sand or gravel, burying the Crabs (Portunidae--Plate XIII) of our own coasts have been found to use the paddle-shaped last pair of legs for digging as well as for swie 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 froh channels that open on either side of the mouth-frame

[Illustration: _PLATE XIV_

_Corystes cassivelaunus_ MALE (ON LEFT) AND FEMALE (ON RIGHT)

BRITISH (REDUCED)

_Albunea symnista_, ONE OF THE HIPPIDEA INDIAN SEAS (REDUCED)]

A more co is found in the Masked Crab (_Corystes cassivelaunus_--Plate XIV) This Crab is co in moderately deep water wherever the bottolish name froive it a grotesque reses, for thevery long, slender chelipeds, while those of the feanization, however, have to do with its habit of burrowing in sand The antennae, which inas 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 antennae are brought together parallel with each other, the hairs interlock and for tube At its base this tube communicates with a space in front of the ill 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 hind deeply into the sand; the body of the Crab is thus forcibly pulled doards 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 accu 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 antennae The antennal tube keeps open a channel leading from the buried Crab to the water above Since this tube coh which the water passes _out_ froill chamber in most Crabs, it was assumed by the older observers that the antennal tube served to carry the outfloater 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 nor drawn _down_ the antennal tube, into the gill chas 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 ular instead of square, being produced forwards between the eyes, and the third ated In this way the exhalent channels carrying the water froin of the head, and are exposed even when the Crab is buried

In the different fas 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 becoed by sediment

The members of the tribe Hippidea (so the Anomura, have habits somewhat similar to those of the Crabs just described They are colobe, and they burroith great rapidity by s The carapace is generally smooth and oval, and the body is co 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 antennae, as in _Corystes_, but by the antennules, so that it affords a striking example of the independent evolution of siins

_Hippa emerita_, which is found on the coasts of North and South America, has the ; and it is stated by Professor S I Smith that the ani 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 _Reht by a bait of Crab at the end of a line, pouncing on it with its sharpitself 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 aredeep into the sand Soory 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

[Illustration: FIG 38--_Callianassa stebbingi_ (FEMALE), A SAND-BURROWING THALassINID FROM THE SOUTH COAST OF ENGLAND NATURAL SIZE]

Nearly all the Thalassinidea (Fig 38) live in burrows, often of considerable depth, in sand or h now classed with the Anoenerally with short carapace and long, soft abdomen They have usually very small eyes, which appear as if they were not of s are short, and carried folded against the sides of the body, probably for use when the ani up or down in its burrow