PHYLUM ARTHKOPODA 



233 



number of segments is variable, and the appendages are not 

 so highly differentiated as in the Malacostraca. A carapace 

 developed from the head is often present. There is no gas- 

 tric mill, and the renal organs are not antennary glands, but 



Fig. 134. — Three stages in the development of Apus. fi, frontal sensory organ; 

 L, digestive gland; s, carapace; 1-4, cephalic appendages; I-XIII, body seg- 

 ments and appendages. (From Lang's Comparative Anatomy.) 



shell-glands opening at the bases of the second maxilla?. The 

 larva nearly always leaves the egg as a characteristic form 

 called the Naitplius (Fig. 134, A), which occurs also, though 



