218 THE LIFE OF CRUSTACEA 



degenerate in their structure. The carapace and the 

 rest of the exoskeleton, no longer needed for protec- 

 tion, 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 pro- 

 duced by the female are very numerous, and the 

 abdomen is very broad and deeply hollowed for their 

 reception. 



While most of the Pinnotheridae 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 Cymo- 

 thoidae 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 



