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 qfydamen. 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 

 epimera (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, 



, 7 . , FIG. 201. Types of crayfish 



as a branchiostegite. There are appendages. A, foiiaceous type, 

 six segments and a terminal exten- second maxilla. 1-4, basopodite ; 



, i . , A i ij 5, endopodite; 6, scaphognathite; 



sion, thejelson,, in the abdomen. ^. ; e pipodite. B, biramous type, 



APPENDAGES. Each Segment swimmeret. ex, bs., protopodite ; 

 i , . . L , ex., exopodite ; en., endopodite. 



bears a pair of jointed appendages c, uniramous type, second walk- 

 which in most cases differ from in s lg g- CX P> b P> protopodite; 



,1,1.. . ip, mp, cp, pp, dp, segments of 



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

 function, but all are probably C ' from the Cambridge Natural 



. ,. r 7 . 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 



