198 "PINNA AND HER CANCER FRIEND." 



Linnaeus. In this species the length of the shell is con- 

 siderably greater than its breadth, and of an oval form, 

 with a central ridge which represents the nose, lateral 

 depressions for the eyes, and a transverse line, bordered 

 by broad but shallow ridges, for the mouth and lips. 

 In some specimens the parts of this face are much more 

 prominent than in others. The species is otherwise 

 remarkable for the great length of its claws and of its 

 antennae. It generally frequents deep water, but is 

 occasionally cast on shore. 



I must not omit to notice two species of minute 

 Crabs, whose curious semi-parasitical habits have long 

 rendered them famous. The readers of Darwin's fan- 

 tastic poem must be familiar with the history of 



" Pinna and her Cancer friend." 



It was known to the ancients that a minute Crab 

 sought refuge in the shell of the Pinna, and modern re- 

 search has detected others which take up their residence 

 within the shells of several other kinds of bivalves, espe- 

 cially of Mussels and Cockles. These little Crabs belong 

 to the genus Pinnotheres, and two species are found on 

 our coast. One of them, P. pisum, is very commonly 

 found within the shell of the Common Mussel, especially 

 when raised from deep water. Its shell is from a quar- 

 ter to nearly half an inch in breadth, rounded and 

 convex, of a thin substance and brownish colour, with 

 one or two yellow spots. The other species, P. veterum, is 

 usually found within the shell of the Pinna, and differs 

 in being of a more angular form and uniform brown 

 colour. The habits of both species appear to be similar. 



