Chap. XII.] THE CRA YFISH. 



2. The Abdomen, behind the cephalo- thorax ; jointed, com- 

 posed of six divisions or somites. 



3. The Telson, forming the posterior end of the body in the 

 mid-line ; divided transversely into two parts, on the under 

 side of the first of which is the vent. 



4. The Appendages, of which there are 20 pairs; 6 being 

 attached to the somites of the abdomen, and 14 to the cephalo- 

 thorax. Of these 9 pairs are visible in Fig. 73, A., viz., eye- 

 stalks, short two-whipped antennules, antennae, pincers, 4 pairs 

 of legs, and the large posterior swimmerets, which, with the 

 telson, constitutes the terminal tail-flap. 



Seen from the side in the normal position of rest, the abdominal 

 region is slightly curved downwards, while the anterior end of 

 the sternal surface is bent upwards, giving rise to what is termed 

 the cephalic flexure. Hence the curve of the ventral or appendage- 

 bearing surface is as represented in Fig. 73, B. 



On the dorsal (or tergal) surface of the cephalo-thorax there is 

 a curved cervical groove, from which there passes backwards on 

 either side of the median line a short longitudinal branchio- 

 cardiac groove. On the tergal surface of each abdominal segment 

 there is a smooth anterior portion, seen best when the abdomen 

 is in flexion or curved up beneath the body, which marks where 

 the segment in front overlaps it. 



Fig. 73, C., shows a posterior view of the fourth abdominal 

 somite, from which the limbs and the soft parts have been 

 removed. The tergal surface (te.\ is, in a dorsal view, broadest 

 in the mid-line, narrowing away to the pleura at each side. 

 The sternum (st.) is very narrow, forming a mere transverse 

 ventral bar. On each side of the sternum is seen the articular 

 cavity (ar.) for the swimmeret. The projecting flaps at the 

 sides are the pleura (pi). Between the articulation of the limb 

 and the pleuron is a narrow area called the epimeron (ep.). 



The somites of the abdomen articulate together by means of 

 pegs or pivots which fit into corresponding depressions, the peg 

 being on the anterior edge of one segment and the socket on 

 the posterior edge of the next. The pleura overlap, especially 

 in flexion. The pleura of the second abdominal somite overlap 



