222 SYSTEMATIC ZOOLOGY. 



SUBCLASS I. 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 with others parasitic habits have resulted 

 in such changes that there is little external resemblance 

 to a crayfish or a crab. 



The Entomostraca include the Phyllopoda, Copepoda, 

 Ostracoda, and Cirripedia. 



ORDER I. PHYLLOPODA. 



These, the lowest of the Crustacea, receive their name 

 from the leaf-like character of the thoracic feet. Some, 

 like Branchipus, are shrimp-like in appearance; others 

 (Apus) have a broad shield over a part of the body, while 

 still others have the body enclosed in a bivalve shell, 

 much like that of a clam. Most of the species inhabit 

 fresh water; a few live in brine. Branchipus, the fairy- 

 shrimp, is common in snow-pools in the northern states. 

 Its eggs require to be dried by the summer sun before 

 they will develop. 



ORDER II. COPEPODA. 



The Copepoda include two groups of forms. The one 

 includes both fresh and salt-water forms of regular crus- 

 tacean appearance (fig. 50), while the other contains 

 forms, commonly known as fish-lice, which are parasitic 

 on fishes, and which as a result have in some cases degen- 

 erated to such an extent that, without a knowledge of 

 their history, one would hardly suspect them of being 

 crustaceans were it not for their young. In their history 



