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From remote antiquity it has been known that a little Crab ([Fig. 69]) is frequently found living within the shells of bivalve Molluscs, such as Oysters, Mussels, and especially the large mussel-like Pinna, which is common in the Mediterranean. Ancient writers regarded this as a case of association for mutual advantage, believing that the Pinnotheres warned the Pinna of the approach of enemies or of the entrance of prey between its gaping valves. It is even stated that the Pinna and Crab were depicted in Egyptian hieroglyphics to symbolize the dependence of a man on his friends.

As a matter of fact, however, there is no reason to believe that the Molluscs which harbour species of Pinnotheres and allied genera benefit in any way by the presence of the Crabs. The latter probably feed, as their hosts do, on particles brought in by the current of water entering the mantle cavity. They are therefore strictly "commensals," though it is usual, and perhaps equally correct, to speak of them as "parasites." The case is, indeed, an example of the difficulty of defining these two terms. At all events, the Pinnotherid Crabs show one of the characteristics of parasites in being to some extent degenerate in their structure. The carapace and the rest of the exoskeleton, no longer needed for protection, have become soft and membranous, and the eyes and antennules, the chief organs of sense, are very minute. As in many parasites, also, the eggs produced by the female are very numerous, and the abdomen is very broad and deeply hollowed for their reception.

Fig. 70—Cirolana borealis. About Twice Natural Size. (After Sars.)

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While most of the Pinnotheridæ live in bivalve Molluscs, some species are associated with other invertebrate animals. Pinnaxodes chilensis is found in a species of Sea-urchin (Strongylocentrotus gibbosus) on the coast of Chili. On opening the shell of the Urchin, the Crab is found enclosed in a thin-walled bag formed by enlargement of the terminal part of the host's intestine.

It did not escape the notice of Aristotle that a little Shrimp sometimes occurred in the Pinna in place of the Crab. This is Pontonia custos, and other species of the same and allied genera have similar habits.

The order Isopoda includes a very large number of parasitic species. The extensive family Cymothoidæ presents a whole series of gradations in habits and structure between actively swimming predatory species and others which in the adult state are permanently fixed to their host, usually a fish, and are incapable of movement. At one end of the series are the species of Cirolana, which have powerful biting jaws. Of C. borealis ([Fig. 70]), Mr. Stebbing remarks that "it is a good swimmer, tenacious of life, a savage devourer of fish, and not to be held in the human hand with impunity." The species is not uncommon in British seas, and numerous individuals will sometimes attack a Cod or other large fish, perhaps after it has been caught on a hook, and gnaw their way into its body, so that when brought to the surface the fish consists of little more than skin and bone.