PHYLUM ARTHROPODA 



199 



Ostracoda 

 (Ostracod) 



Anostraca 

 (Fairy shrimp) 



Figure 110. Representative crustaceans. The lines suggest possible relationships. The figures 

 are not drawn to scale. 



Regions of the body 



The body consists of two distinct regions, 

 an anterior rigid portion, the cephalothorax, 

 and a posterior series of segments, the abdo- 

 men. The entire body is segmented, but the 

 joints, except one, have been obhterated on 

 the dorsal surface of the cephalothorax. 



Exoskeletal structures of a segment 



A typical segment consists of a convex 

 dorsal plate, the tergum, a ventral transverse 



bar, the sternum, and plates projecting down 

 at the sides, the pleura (Fig. 113). 



Cephalothorax 



The cephalothorax consists of segments 

 1-12,* which are enclosed dorsally and later- 



* Many textbooks give 1 3 segments; but accord- 

 ing to Snodgrass and other authorities, the anten- 

 nules of arthropods are developmentally and phylo- 

 genctically different from the appendages posterior 

 to them. The antennules arise from a structure 

 which appears to be a homologue of the prostomium 



