A SWIMMING CRAB, Portunus depurator. BRITISH. (REDUCED)

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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 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.