PHYLUM ARTHROPODA 



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. fs, frontal sensory organ; 

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

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



shelf-glands opening at the bases of the second maxillae. The 

 larva nearly always leaves the egg as a characteristic form 

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



