202 LIFE BY THE SEASHORE. 



among which may be specially mentioned P. depurator, the 

 wrinkled swimming crab, and P. marmoreus, a form with 



beautifully marbled 

 carapace. The 

 species are so nu- 

 merous that it seems 

 unnecessary here to 

 give their distin- 

 guishing features, 

 especially as many 

 of them are very 

 local in their distri- 

 bution. Swimming 

 crabs are most fre- 

 quently found on 

 the shore thrown up 



Fm. 57. Portunus depurator, the wrinkled swimming V>v thp wavp<3 hnfc 

 crab. Note the shape of the last pair of legs. . Dy * 8 . waves > P ut 



in certain localities, 



especially at the edge of rocks running out into the clear 

 sand, it is not uncommon to find them in the living active 

 condition. In the sandy pools the peculiar method of loco- 

 motion may then be readily observed. A careful anatomical 

 comparison with Carcinus should also be made. 



Of the last family of crabs, the Catometopa, or quadri- 

 lateral crabs, one example only need be described. This is 

 the very curious Pinnotheres pisum, the pea-crab, a very 

 small crab found inside the shells of many bivalves, 

 especially the horse-mussel (Mytilus modiolus), the oyster 

 (Ostrea), the cockle (Cardium), and others. The peculiar 

 habit has given rise to many curious superstitions, the 

 present being one of the first cases known of what we 

 now call " commensalism." The carapace is arched, almost 

 circular, smooth and delicate, and in the female almost 

 uncalcified. The males are smaller than the females, and 

 have a projecting forehead, while that of the females is 

 uniformly rounded. 



This concludes our brief survey of the true crabs, which, 

 as we have seen, are the most specialised of the Decapod 

 Crustacea. We have begun our survey of the Crustacea 

 with the Decapods because they are the largest and most 

 conspicuous forms, and because they illustrate so admirably 



