PARASITES AND MESSMATES 217 



the shells of bivalve Molluscs, such as Oysters, Mus- 

 sels, 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 



FIG. 69 THE COMMON PEA CRAB (Pinnotheres pisum), FEMALE. 

 NATURAL SIZE. 



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 



