THE ARTHROPOD A 



315 



rapidly forward, thus sending the animal's body backward from the posi- 

 tion it occupied. 



A typical segment of the abdomen (Fig. 204) consists of the upper 

 portion called the tergum ( ), a ventral portion, 



the .sternum, two pleura (the extended portions continuing ventrally be- 

 hind the sternum), and two epimera, these latter forming the roof which 

 extends from the pleura to the appendage. 



A. Diagram of skeleton of an abdominal segment of Astacus. 

 bs. Basipodite. ex. Coxopodite of swimmeret. ep. Epimeron. jt. 

 Point of articulation with skeleton of adjacent segment. pi. 

 Pleuron. st. Sternum, tg. Tergum. (After Latter.) 



B. Section through cephalothorax of a crab. (After Pearson.) 

 H., Heart ; Te., extension of the tergum ; ST., sternum ; PL., 

 pleuron ; T., tendons ; 1st W. L., insertion of first walking leg ; Br., 

 gill in gill-chamber ; g., gut ; d.a., descending artery ; A., afferent 

 branchial ; E., efferent branchial. 



There are thirteen segments in the cephalothorax. The eyes are 

 not counted as appendages. A cervical groove forms the separating line 

 between head and thorax. The entire dorsal shield of the cephalothorax 

 is called the carapace (. ), the jointed end extending 



between the eyes being known as the rostrum, while the portion on the 

 sides covering the gills are the branchiostegites ( ). 



The entire crayfish possesses twenty segments, counting telson and uro- 

 pods as one. 



Each pair of the appendages is slightly different in appearance from 

 any other pair, though there is much similarity between them. The 

 three distinguishing types of crayfish appendages are known as (1) 

 foliaceous ( ), (second maxilla); (2) biramous 



( ), (swimmerettes) ; (3) uniramous ( ), 



(walking legs). 



The female has an. opening at the base of the third walking leg 

 through which eggs are exuded. She also possesses a single opening 

 in the midline through which sperm may be inserted. Immediately be- 



