Fig. 335. Corystes Cassivelaunus, male.
The crabs are numerous in species and various in size. The long-clawed crab (Corystes Cassivelaunus) of Pennant and Leach (Fig. 335) is remarkable for its long antennæ, which considerably exceed the body. The jaw-feet have their third joint longer than the second, terminating in an obtuse point, with a notch on its interior edge; eyes wide apart, borne upon large peduncles, which are nearly cylindrical and short; anterior feet large, equal, twice the length of the body, and nearly cylindrical in the males; in the females (Fig. 336) about the length of the body, and compressed, especially towards the hand-claw. The other feet terminate in an elongated nail or claw, which is straight-pointed and channeled longitudinally: carapace oblong-oval, terminating in a rostrum anteriorly truncated and bordered posteriorly; the regions but slightly indicated, with the exception of the cordian region, the branchial or lateral regions being very much elongated.
Fig. 336. Corystes Cassivelaunus, female.
Latreille gives the name of Corystes, which signifies a warrior armed, to this genus of Crustaceans, from κόρυς, a helmet, but it is perfectly inoffensive. Pennant had already conferred the name of Cassivelaunus, the chief of the Ancient Britons, for the singular reason, according to Gosse, that the carapace, which is marked by wrinkles, bears, in old males especially, the strongest and most ludicrous resemblance to the face of an ancient man. Pennant's well-known sympathy with his British ancestry certainly never led him to caricature the grand old British warrior, as Mr. Gosse surmises. On the contrary, he saw in the Crustacean a creature armed at all points, and he named it after the hero of his imagination.
In this species the surface of the carapace is somewhat granulous, with two denticles between the eyes, and three sharp points directed forward on each side. The male has only five abdominal pieces, but the vestiges of the separation of the two others may be clearly remarked upon the outer mediate or third piece, which is the largest of all. The length of the antennæ is remarked on by Mr. Couch, in his Cornish Fauna. "These organs," he says, "are of some use beyond their common office of feelers; perhaps, as in some other Crustaceans, they assist in the process of excavation; and, when soiled by labour, I have seen the crab effect their cleaning by alternately bending the joints of their stalks, which stand conveniently angular for the purpose. Each of the long antennæ is thus drawn along the brush that fringes the internal face of the other, until both are cleared of every particle that adhered to them." On the other hand, Mr. Gosse suggests that the office of the antennæ is to keep a passage open for ejecting the deteriorated water after it has bathed and aerated the gills. "I have observed," he says, "that, when kept in an aquarium, these crabs are fond of sitting bolt upright, the antennæ placed close together, and pointing straight upwards from the head. This is doubtless the attitude in which the animal sits in its burrow, for the tips of the antennæ may often be seen just projecting from the sand. When the chosen seat has happened to be so close to the glass side of the tank as to bring the antennæ within the range of a pocket lens, I have minutely investigated these organs without disturbing the old warrior in his meditation. I saw on each occasion that a strong current of water was continuously pouring up from the points of the antennæ. Tracing this to its origin, it became evident that it was produced by the rapid vibration of the foot-jaws drawing in the surrounding water, and pouring it off upwards between the united antennæ, as through a tube. Then, on examining these organs, I perceived that the form and arrangement of their bristles did indeed constitute each antennæ a semi-tube, so that when the pair were brought face to face the tube was complete."
Among the numerous genera of Brachyurous Crustaceans, Grapsus is distinguished by its less regularly quadrilateral form; the body nearly always compressed, and the sternal plastron but little or not at all curved backwards; the front strongly recurved, or, rather, bent downwards; the orbits oval-shaped and of moderate size; the lateral edges of the carapace slightly curving and trenchant; the ocular pedicles large, but short: their insertion beneath the front and the cornea occupies one-half of their length.
Fig. 337. Pagurus Bernhardus. 1, out of the shell; a, right jaw-foot; b, in the shell.