PLATE XIV
Corystes cassivelaunus. MALE (ON LEFT) AND FEMALE (ON RIGHT). BRITISH (REDUCED)
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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 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.