/. 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 the zoea. The nauplius (figs. 7, 429) 

 consists of three segments covered by a dorsal shield and bearing 

 below three pairs of appendages. The first pair, developing later 

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



FIG. 415. Zoea of Cai-cinus mcenas. (After Faxon.) 7t, heart; i, intestine; 1-VIL, 

 cephalic appendages. 



second antennas and mandibles, are schizopodal. Internally there 

 is a three-chambered alimentary tract, a supraoesophageal ganglion 

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

 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 



