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. On the sternal aspect of the thorax (figs. 3 and 39, A) a triangular space is interposed between the basal joints or coxopodites of the penultimate and the ante-penultimate pairs of ambulatory legs, while the coxopodites of the more anterior limbs are closely approximated. The triangular area in question is occupied by two sterna (fig. 39, A, _XII_, _XIII_), the lateral margins of which are raised into flange-like ridges. The next two sterna (_X_, _XI_) are longer, especially that which lies between the forceps (_X_), but they are very narrow; while the lateral processes are reduced to mere tubercles at the posterior ends of the sterna. Between the three pairs of maxillipedes, the sterna (_VII_, _VIII_, _IX_) are yet narrower, and become gradually shorter; but traces of the tubercles at their posterior ends are still discernible. The most anterior of these sternal rods passes into a transversely elongated plate, shaped like a broad arrow (_V_, _VI_), which is constituted by the conjoined sterna of the two posterior somites of the head.