SYNOPSIS OF CALIFORNIA STALK-EYED 

 CRUSTACEA. 



BY SAMUEL J. HOLMES. 



Introduc TION. 



The Crustacea are commonly divided into two princi- 

 pal groups, the Malacostraca and the Entomostraca, 

 although the primary divisions of the latter group may 

 well be, as they often are, considered coordinate with 

 the former. There is such great diversity of form 

 among the Entomostraca that the group is scarcely 

 definable except by negative characters. The number 

 of segments composing the body is very variable and 

 the openings of the genital ducts do not occur in the 

 same segment in all the members of the group. There 

 is no masticatory stomach and corresponding append- 

 ages are modified in most diverse ways in different 

 orders. The Malacostraca, on the other hand, while 

 they include a great variety of forms, constitute a well 

 defined subclass. Except in cases in which it is quite 

 evident that certain segments are fused or lost, the body 

 is composed of twenty (or in Nehalia of twenty-one) 

 segments. The typical number of appendages is nine- 

 teen. There is a pair of compound lateral eyes, and the 

 stomach, except in a few parasitic species, is masticatory. 

 The genital ducts of the male open on the last thoracic 

 segment; those of the female on the antepenultimate 

 one. The head is typically composed of five segments 



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