The legs of the crabs are very differently formed in various species. In those which have been called sea-spiders they are very long, thin, and weak, so that the animal swims badly, and is a slow and uncertain pedestrian. For greater security it therefore generally seeks a greater depth, where, concealed among the sea-weeds, it wages war with annelides, planarias, and small mollusks. Sea-spiders are often found on the oyster-banks, and considered injurious by the fishermen, who unmercifully destroy them whenever they get hold of them.
In other species the legs are short, muscular, and powerful, so as rapidly to carry along the comparatively light body. The tropical land-crabs and the genera Ocypoda and Grapsus, which form the link between the former and the real sea-crabs, are particularly distinguished in this respect.
The Rider or Racer (Ocypoda cursor), who is found on the coasts of Syria and Barbary, and abounds at Cape de Verde, owes his name to his swiftness, which is such that even a man on horseback is said not to be able to overtake him. The West Indian ocypodas dig holes three or four feet deep, immediately above high-water mark, and leave them after dusk. Towards the end of October they retire further inland, and bury themselves for the winter in similar holes, the opening of which they carefully conceal.
American Sand-Crab.
In the Portuni, or true Sea-crabs, finally, we find the hind pair of legs flattened like fins, so that they would cut but a sorry figure on the land, but are all the better able to row about in their congenial element.
Spotted Fin-Crab.
A strange peculiarity of many crabs is the quantity of parasites they carry along with them on their backs. Many marine productions, both of a vegetable and animal nature, have their birth and grow to beauty on the shell of the sea-spider. Corallines, sponges, zoophytes, algæ, may thus be found, and balani occasionally cover the entire upper surface of the body of the crab. "All the examples of the Inachus Dorsettensis which I have taken," says the distinguished naturalist, Mr. W. Thompson of Belfast, "were invested with sponge, which generally covers over the body, arms, and legs; algæ and zoophytes likewise spring from it." In this extraneous matter some of the smaller zoophytes find shelter, and, together with the other objects, render the capture of the Inachus Dorsettensis interesting far beyond its own acquisition. In Mr. Hyndman's collection, there is a sea-spider carrying on its back an oyster much larger than itself, and covered besides with numerous barnacles. Like Atlas, the poor creature groaned under a world.
The extraneous matters which so many crabs carry along with them are, however, far from being always a useless burden; they are often a warlike stratagem, under cover of which the sly crustacean entraps many a choice morsel. Thus Bennett witnessed at Otaheite the proceedings of an interesting Hyas species, which disguised itself by investing its body with a covering of decayed vegetable substances and coral-sand. The better to ensnare its prey, the back was covered with rigid and incurved bristles, calculated to retain the extraneous substances, while the short and well concealed forceps-claws were ready for the attack, and the ophthalmic peduncles, curving upward to raise the eyes above the pile of materials, gave the wily crab the great advantage of seeing without being seen. As soon as an unfortunate mollusk, unsuspicious of evil, approached the lurking ruffian, he darted upon it like an arrow, and, ere it could recover its presence of mind, was busy tearing it to pieces.