Each ring consists of a dorsal, arched portion, called the _tergum_ (fig. 21; fig. 36, p. 142, _t. XIX_), and a nearly flat ventral portion, which is the _sternum_ (fig. 36, _st. XIX_). Where these two join, a broad plate is sent down on each side, which overlaps the bases of the abdominal appendages, and is known as the _pleuron_ (fig. 36, _pl. XIX_). {97} The sterna are all very narrow, and are connected together by wide spaces of flexible exoskeleton. [Illustration: FIG. 21.—_Astacus fluviatilis._—Two of the abdominal somites, in vertical section, seen from the inner side, to show ×, ×, the hinges by which they are articulated with one another (× 3). The anterior of the two somites is that to the right of the figure.] 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 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 {98} marked off by a transverse groove from the rest of the tergum as an _articular facet_. The front edge of the articular facet is continued into a sheet of flexible cuticula, which turns back, and passes as a loose fold to the hinder edge of the overlapping tergum (fig. 21). This tergal _interarticular membrane_ allows the terga to move as far as they can go in flexion; whilst, in extreme extension, they are but slightly stretched. But, even if the intersternal membranes presented no obstacle to excessive extension of the abdomen, the posterior free edge of each tergum fits into the groove behind the facet in the next in such a manner, that the abdomen cannot be made more than very slightly concave upwards without breaking.