THE JOINTS OF THE ABDOMEN. 



97 



The sterna are all very narrow, and are connected 

 together by wide spaces of flexible exoskeleton. 



When the abdomen is made straight, it will be found 

 that these inter sternal membranes are stretched as far 

 as they will yield. On the other hand, when the abdomen 



Fig. 21. — Astacus fluviatilis. — Two of the abdominal somites, in vertical 

 section, seen from the inner side, to show x , x , the hinges by 

 which they are articulated with one another ( x 3). The anterior 

 of the two somites is that to the right of the figure. 



is bent up as far as it will go, the sterna come close 

 together, and the intersternal membranes are folded. 



The terga are very broad ; so broad, in fact, that each 

 overlaps its successor, when the abdomen is straightened 

 or extended, for nearly half its length in the middle 

 line ; and the overlapped surface is smooth, convex, and 



