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.
If many crabs are burdened with small animals and plants, others live parasitically in the shells of mollusks. Thus the small _Pinnotheres veterum_ claims the hospitality of the Pinna, a large bivalve of the Mediterranean. The ancients supposed that this was a friendly connection, an _entente cordiale_, formed for mutual defence: that the Pinna, being destitute of eyes, and thus exposed when he opened his shell to the attacks of the cuttle-fish and other enemies, was warned of their approach by his little lodger, upon which he immediately closed his shell and both were safe. Unfortunately, there is not a word of truth in the whole story. The sole reason why the Pinnotheres takes up its abode under a stranger's roof is the softness of its own integuments, which otherwise would leave it utterly defenceless; nor does the Pinna show the least sign of affection for its guest, who, on returning from an excursion, often finds it very difficult to slip again into the shell.
According to Mr. Thompson, the _Modiola vulgaris_, a species of mussel very common on the Irish coast, almost always harbours several parasitic crabs (_Pinnotheres pisum_). At Heligoland, Dr. Oetker, to whom we are indebted for the best work on that interesting island, scarce ever found a modiola without several guests of this description, while he never could find any in oysters, mussels, and other nearly related species. What may the reason be of either this predilection or that desertion?