On examination of the sternal aspect of the cephalothorax the signs of division into somites become plain (figs. 3 and 39, _A_). Between the last two ambulatory limbs there is an easily recognisable sternum (_XIV._), though it is considerably narrower than any of the sterna of the abdominal somites, and differs from them in shape. The deep transverse fold which separates this hindermost thoracic sternum from the rest of the sternal wall of the cephalothorax, is continued upwards on the inner or epimeral wall of the branchial cavity; and thus the sternal and the epimeral portions of the posterior thoracic somite are naturally marked off from those of the more anterior somites. [Illustration: FIG. 38.—_Astacus fluviatilis._—The mode of connexion between the last thoracic and the first abdominal somites (× 3). _a_, L-shaped bar; _cpe_, carapace; _cxp. 14_, coxopodite of the last ambulatory leg; _plb._, place of attachment of the pleurobranchia; _st. XV_, sternum, and _t. XV_, tergum of the first abdominal somite.] The epimeral region of this somite presents a very curious structure (fig. 38). Immediately above the articular cavities for the appendages there is a shield-shaped plate, the posterior, convex edge of which is sharp, prominent, and setose. Close to its upper boundary the plate exhibits a round perforation (_plb._), to the margins of which the stem of the hindermost {151} pleurobranchia (fig. 4, _plb. 14_) is attached; and in front of this, it is connected, by a narrow neck, with an elongated triangular piece, which takes a vertical direction, and lies in the fold which separates the posterior thoracic somite from the next in front. The base of this piece unites with the epimeron of the penultimate somite. Its apex is connected with the anterior end of the horizontal arm of an L-shaped calcified bar (fig. 38, _a_), the upper end of the vertical arm of which is firmly, but moveably, connected with the anterior and lateral edge of the tergum of the first abdominal somite (_t. XV._). The tendon of one {152} of the large extensor muscles of the abdomen is attached close to it. 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.