CRUSTACEA. 209 



closely like the adult in all except size. Growth is allowed 

 for by frequent molts of the external cuticle of the body. 



Over 10,000 species of Crustacea are known, almost all 

 of them aquatic, and the majority marine. Only a few, 

 like the sow-bugs and land-crabs, live on the land. A few 

 are vegetarians, some are parasites on other animals, but 

 the majority are scavengers, feeding on decaying organic 

 matter. The Crustacea may be conveniently divided into 

 two "subclasses": Malacostraca and Entomostraca. 



SUBCLASS I. MALACOSTRACA. 



This group contains the larger and higher Crustacea, in 

 which the body consists of twenty somites,* all of which 

 except the last (telson) may bear appendages. Compound 

 eyes are usually present ; and the nauplius stage (p. 208) is 

 usually passed in the egg. Besides several unimportant 

 groups, this subclass contains the orders Decapoda and 

 Tetradecapoda. 



ORDER I. DECAPODA (p. 164). 

 ORDER II. TETRADECAPODA (p. 168). 



SUBCLASS II. ENTOMOSTRACA. 



This division contains a large number of forms, mostly 

 small, or even microscopic in size. The number of body- 

 segments is usually less than twenty, but occasionally there 

 may be many more. Some are decidedly shrimp-like in 

 form, but in others parasitic habits have resulted in such 

 changes that there is little external resemblance to a cray- 

 fish or a crab. In fact, this degeneration may go so far in 



* Twenty-one in Nebalia. 



