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. 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.