THE CRUSTACEA OF NEW JERSEY. 43 



Key to the sub-classes. 



a. Size small, often microscopic ; post-cephalic segments variable, and ap- 

 pendages comparaively little differentiated ; large cephalic carapace often 

 present; considerable portion of enteric canal derived from mesenteron, 

 and no gastric mill ; excretory organs shell-glands ; metamorphosis pres- 

 ent, larva usually leaves egg as free-swimming nauplius. 



ENTOMOSTRACA 



aa. Size usually large; usually (except one order) thorax of eight and 

 abdomen of seven segments, and appendages usually highly differentiated ; 

 usually mesenteron forms only small portion of adult enteric canal, and 

 gastric mill present; excretory organs antennary glands; nauplius stage 

 usually passed through in egg, though more or less complex meta- 

 morphosis. MALACOSTRACA 



Sub-Class ENTOMOSTRACA. 



The Entomostracans. 



Body often covered with a shell or carapace, this either leathery 

 or horny in structure, composed of one or two pieces, sometimes 

 with the appearance of a bivalve shell, in other forms like a 

 buckler which largely or completely envelops the animal. Num- 

 ber of segments behind head varies. Considerable portion of 

 the enteric canal derived from the mesenteron, and no gastric 

 mill. The excretory organs are shell glands. Branchiae attached 

 either to feet or organs of mastication. Feet jointed, and all 

 more or less ciliated, and usually of more or less similar appear- 

 ance. Body regularly moulted or completely changing the shell 

 with growth, this sometimes a form of metamorphosis. 



These crustacea are of relatively simple organization, and 

 mostly small, the vast majority in fact being microscopic. 



Key to the orders. 



a. Body distinctly segmented, covered by cephalic carapace; post-cephalic 

 appendages leaf-like. phyllopoda 



aa. Body unsegmented, with rudimentary abdomen, mostly enclosed in 

 carapace or shell of bivalve form; appendages in seven pairs. 



OSTRACODA. 



