PHYLUM ARTHROPODA 



277 



Anatomy and Physiology. — External Features. — The cray- 

 fish is a segmented animal, but the joints have been obliterated 

 on the dorsal surface of the ante- 

 rior end. The body shows two 

 distinct regions, an anterior rigid 

 portion, the cephalothorax, and a 

 posterior flexible abdomen. A 

 chitinous exoskeleton, impregnated 

 with lime salts, supports and pro- 

 tects the soft parts of the body. 



A typical segment (Fig. 200) 

 consists of a tergum (T),a, sternum 

 (S), two pleura (PL), and two 

 e pirn era (EP). The cephalo- 

 thorax includes segments I— XIII; 

 a cervical groove separates the 

 cephalic or head region from the 

 thoracic region. The dorsal 

 shield of the cephalothorax is 

 called the carapace; its anterior 

 pointed extension is known as 

 the rostrum, and the heavy flap 

 on either side protecting the gills, 

 as a branchiostegite. There are 

 six segments and a terminal exten- 

 sion, the telson, in the abdomen. 



Appendages. — Each segment 

 bears a pair of jointed appendages 

 which in most cases differ from 



... , ip, mp, cp, pp, dp, segments of 



the other pairs in structure and endopodite; ep., epipodite. (A and 

 function, but all are probably c - from the Cambridge Natural 



History ; B, from Lankester's 



variations of a biramous type Treatise.) 



(Fig. 200) consisting of a basal 



protopodite (P), an inner branch, the endopodite (N), and an 



outer branch, the exopodite (X). Three types of appendages can 



Fig. 201. — Types of crayfish 

 appendages. A, foliaceous type, 

 second maxilla. 1-4, basipodite ; 

 5, endopodite; 6, scaphognathite; 

 ep., epipodite. B, biramous type, 

 swimmeret. ex, bs., protopodite ; 

 ex., exopodite ; en., endopodite. 

 C, uniramous type, second walk- 

 ing leg. cxp, bp, protopodite ; 



