160 GENERAL ZOOLOGY 



The Common Prawn. The prawn (Palsemone'tes vulgaris, 

 frontispiece), is found abundantly among seaweeds near 

 shore in sea water and also in brackish and almost fresh 

 water. It is a good example of a pelagic, or surface-inhabit- 

 ing, animal of considerable size, which is so nearly transpar- 

 ent that it may be overlooked unless it moves. The prawn, 

 or, as it is frequently called, the shrimp, is about two inches 

 long and resembles in general form the lobster or the cray- 

 fish. The body is, however, more compressed (flattened from 

 side to side) and more strongly arched from head to tail. 

 The carapace is thin but tough and leathery, and covered 

 with many reddish-brown dots. It is so transparent that 

 the stomach and intestine can be seen clearly while the an- 

 imal is at rest. 



The Edible Shrimp. A close relative of the prawn is the 

 shrimp (Cran'gon vulga'ris), the tail of which is very much 

 sought as food. This species is very widely distributed, for 

 it lives in shallow water on both the Atlantic and Pacific 

 coasts of North America, and the same species occurs in 



Europe. 



Crabs. The species represented in Fig. 87 (Pagu'rus 

 pollica'ris) is called the hermit crab, probably because it 

 lives in a " house* ' by itself. The house consists of the shell 

 of a dead snail, which may have been washed to the beach, 

 or may have rested on the bottom of the bay. The hermit 

 crab in its early life is pelagic, but at a certain stage of its 

 development it sinks to the bottom, finds a snail shell, and 

 backs into it ; from that time on, the general shape of the 

 body and the special modification of certain organs are de- 

 termined by the form of the snail shell. The abdomen is 

 soft, and all the abdominal appendages except the terminal 

 ones are misshapen and useless. The terminal appendages 

 extend laterally like flanges, and prevent the body from 

 being drawn forcibly from the snail shell by an enemy. 



