ARTHROPODA 



[49 



hinged on the dorsal side. The outer covering of the Entomo- 



straca is generally not calcareous, but a thin, flexible, chitinous 

 layer. The alimentary canal is a very simple tube, a point in 

 which this class differs strikingly from the next. Finally, in 

 those Entomostraca which develop with a larval stage, the 

 embryo leaves the egg in what is called the nauplius sta 

 This larva (Fig. 140) is oval in outline, with a simple unpaired 



Fk;. 140. Three stages in the development of Apus. ft, frontal sensory organ ; /., digestive 

 gland; s, carapace ; 1-4, cephalic appendages; I-XIII, body-segments and append; 



I I- 10111 Lang's Comparative Anatomy.) 



median eye, three pairs of appendages, the first simple, the 

 second and third biramous like the typical crustacean append- 

 age, and posterior to the appendages are traces of segmentation. 

 Several ecdyses metamorphose the larva into the adult. There 

 are four orders in this class, and all are in general sufficiently 

 different externally to be readily distinguishable. 



