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 intersternal 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 
H 
