CRUSTACEA IN GENERAL 139 



little animals which run about sideways, moving one of their 

 pinchers, which is larger than the other, in the manner of a fiddle 

 bow. 



The hermit crab (Fig. 78) is not satisfied with the protection 

 afforded by its exoskeleton, but searches about until it finds an 

 empty snail shell, into which it inserts its abdomen. This pro- 

 tecting shell is then carried about until the hermit crab has out- 

 grown it, when it is cast off and a new and larger one found. 

 Often the shell becomes covered with a colony of polyps. These 

 polyps are transported from place to place by the crab, and in 



Fig. 70. — A common shrimp. (From Davenport.) 



return they pay their fare by stinging any of the crab's enemies 

 that may attack it. Such a relation is similar to that described 

 between the plant lice and the ants (p. 42) and is known as 

 commensalism {con, together; mensa, table), meaning living at 

 the same table. 



Shrimps (Fig. 79) and prawns are long-tailed decapods that 

 resemble the crayfish; they are important as a food supply for 

 man. 



Barnacles. — The barnacles (Fig. 80) are marine Crustacea 

 that were for a long time placed in the same group with the 

 oysters because of their shell. A study of their life history, 

 however, proved them to be Crustacea. The young when they 

 hatch from the egg look something like young crayfishes. When 

 they have reached a certain size, they attach themselves to rocks, 

 whales, turtles, or the bottoms of ships, and form a shell about 

 themselves. Here they spend the rest of their lives drawing 



