/. CRUSTACEA. 



413 



either suppressed or, as is easily shown, the corresponding stages 

 are passed in the egg. Two of the larval stages are especially im- 

 portant, the nauplius and tlie zoea. The ncmplius (figs. 7, 439) 

 consists of three segments covered by a dorsal shield and bearing 

 below three pairs of appendages. The first -pah, developing later 

 to the first antennje, are simple; the others, corresponding to the 



Fig. 



41.5.— Zoea of Carcinu& mcenan. (After Faxon.) 

 cephalic appendages. 



7i, heart; t, intestine; I-VII, 



second antennas and mandibles, are schizopodal. Internally there 

 is a three-chambered alimentary tract, a suprauesoj^hageal ganglion 

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

 is almost universal among the lower Crustacea, and some writers 

 believe that it represents an ancestral form from which the Crus- 

 tacea have descended, a view open to much objection. 



The zoea is more complex. It consists (fig. 415) of cephalo- 

 thorax and abdomen, the latter without appendages, the former 



