I. CRUSTACEA 



303 



shield and bearing three pairs of appendages. The first pair, developing 

 later to the first antenna, are simple; the others, corresponding to 

 the second antenna; and mandibles, are schizopodal. Internally 

 there is a three-regional alimentary tract, a supracesophageal 

 gangUon on which is an unpaired eye, and a ventral chain. The nauplius 

 is almost universal among the lower Crustacea, and some believe that it 

 represents an ancestral form from which the Crusta- 

 cea have descended, a view open to much objection. 



The zoea consists (fig. 379) of cephalothorax and 

 abdomen, the latter without appendages, the former 

 with several pairs of schizopodal swimming feet. 

 There are two large compound eyes and, dorsal to 

 the intestine, a heart. Frequently the carapace is 

 armed with very long 

 spines projecting from 

 front, back, and sides, 

 which are intended as pro- 

 tection from enemies. 



Both nauplius and zoea 

 rarely appear in the life 

 cycle of one individual. 

 The nauplius is charac- 

 teristic of the lower Crusta- 

 cea — the 'Entomostraca,' 

 and appears in only a few 

 Malacostraca, like the 

 schizopods and Peneus, 

 and there precedes the 



zoea stage. The zoea, on mamas (after Faxon), h, heart; 

 , I, intestine, I-VII, cephalic ap- 



the Other hand, has never pendages. 



been noticed in the Ento- 

 mostraca, but occurs in many Malacostraca. It must 

 not be forgotten that many forms among both Entomostraca and 

 Malacostraca have no zoeal or nauplius stage. Frequently the lower 

 Crustacea are united under the name Entomostraca, but, aside from the 

 nauplius stage and the possession of a shell gland, the only characters 

 of the group are negative. 



Suh Class I. TrilobitcF. 



The most important crustacean fossils are the Trilobites which 

 appeared in the Cambrian and died out in the Permian, being extremely 



