Pea-Crab.
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.
Pinna Augustana.
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?
The numerous family of the Paguri, or Hermit crabs, is also condemned by its formation to lead a parasitic and robber-life. The fore part of the body is indeed, as in other crabs, armed with claws and covered with a shield, but ends in a long soft tail provided with one or two small hooks. How then are the poor creatures to help themselves? The hind part is not formed for swimming, and its weight prevents them from running. Thus nothing remains for them but to look about them for some shelter, and this is afforded them by several conchiform shells, buccina, neritæ, in which they so tenaciously insert their hooked tails, as if both were grown together. So long as they are young and feeble, they content themselves with such shells as they find empty on the strand, but when grown to maturity, they attack living specimens, seize with their sharp claws the snail, ere it can withdraw into its shell, and after devouring its flesh, creep without ceremony into the conquered dwelling, which fits them like a coat when they take a walk, and the mouth of which they close when at rest with their largest forceps, in the same manner as the original possessor used his operculum or lid. How remarkable that an animal should thus find in another creature belonging to a totally different class, the completion, as it were, of its being, and be indebted to it for the protecting cover which its own skin is unable to secrete!
Diogenes Hermit Crab.