The sternum and the shield-shaped epimeral plates constitute a solid, continuously calcified, ventral element of the skeleton, to which the posterior pair of legs is attached; and as this structure is united with the somites in front of and behind it only by soft cuticle, except where the shield-shaped plate is connected, by the intermediation of the triangular piece, with the epimeron which lies in front of it, it is freely movable backwards and forwards on the imperfect hinge thus constituted.
In the same way, the first somite of the abdomen, and, consequently, the abdomen as a whole, moves upon the hinges formed by the union of the L-shaped pieces with the triangular pieces.
In the rest of the thorax, the sternal and the epimeral regions of the several somites are all firmly united together. Nevertheless, shallow grooves answering to folds of the cuticle, which run from the intervals between the articular cavities for the limbs towards the tergal end of the inner wall of the branchial chamber, mark off the epimeral portions of as many somites as there are sterna, from one another.
[Illustration: FIG. 39.—_Astacus fluviatilis._—The cephalothoracic sterna and the endophragmal system (× 2). _A_, from beneath; _B_, from above. _a_, _a′_, arthrophragms or partitions between the articular cavities for the limbs; _c.ap_, cephalic apodeme; _cf_, cervical fold; _epn. 1_, epimeron of the antennulary somite; _h_, anterior, and _h′_, posterior horizontal process of endopleurite; _lb_, labrum; _m_, mesophragm; _mt_, metastoma; _p_, paraphragm; _I–XIV_, cephalothoracic sterna; _1–14_, articular cavities of the cephalothoracic appendages. (The anterior cephalic sterna are bent downwards in A so as to bring them into the same plane with the remaining cephalothoracic sterna; in B these sterna are not shown.)]
A short distance above the articular cavities a transverse groove separates a nearly square area of the lower part of the epimeron from the rest. Towards the anterior and upper angle of this area, in the two somites { 153} which lie immediately in front of the hindermost, there is a small round aperture for the attachment of the { 154} rudimentary branchia. These areæ of the epimera, in fact, correspond with the shield-shaped plate of the hindermost somite. In the next most anterior somite (that which bears the first pair of ambulatory legs) there is only a small elevation in the place of the rudimentary branchia; and in the anterior four thoracic somites nothing of the kind is visible.