Anteriorly to this, and between it and the posterior end of the elongated oral aperture, the sternal region is {155} occupied only by soft or imperfectly calcified cuticle, which, on each side of the hinder part of the mouth, passes into one of the lobes of the metastoma (_mt_). At the base of each of these lobes there is a calcified plate, united by an oblique suture with another, which occupies the whole length of the lobe and gives it firmness. The soft narrow lip which constitutes the lateral boundary of the oral aperture, and lies between it and the mandible, passes, in front, into the posterior face of the labrum (_lb_). In front of the mouth, the sternal region which appertains, in part, to the antennæ, and, in part, to the mandibles, is obvious as a broad plate (_III_), termed the _epistoma_. The middle third of the posterior edge of the epistoma gives rise to a thickened transverse ridge, with rounded ends, slightly excavated behind, and is then continued into the labrum (_lb_), which is strengthened by three pairs of calcifications, arranged in a longitudinal series. The sides of the front edge of the epistoma are excavated, and bound the articular cavities for the basal joints of the antennæ (_3_); but, in the middle line, the epistoma is continued forwards into a spear-head shaped process (figs. 39 and 40, _II_), to which the posterior end of the antennulary sternum contributes. The antennulary sternum is very narrow, and its anterior or upper end runs into a small but distinct conical median spine (fig. 40, _t._). Upon this follows an uncalcified plate, bent into the form of a half cylinder (_I_), which lies between the inner ends of {156} the eye-stalks and is united with adjacent parts only by flexible cuticle, so that it is freely movable. This represents the whole of the sternal region, and probably more, of the ophthalmic somite. [Illustration: FIG. 40.—_Astacus fluviatilis._—The ophthalmic and antennulary somites (× 3). _I_, ophthalmic, and _II_, antennulary sternum; _1_, articular surface for eyestalk; _2_, for antennule; _epm_, epimeral plate; _pcp_, procephalic process; _r_, base of rostrum; _t_, tubercle.] The sterna of fourteen somites are thus identifiable in the cephalothorax. The corresponding epimera are represented, in the thorax, by the thin inner walls of the branchial chamber; the pleura, by the branchiostegites; and the terga, by so much of the median region of the carapace as lies behind the cervical groove. That part of the carapace which is situated in front of this groove occupies the place of the terga of the head; while the low ridge, skirting the oral and præ-oral region, in which it terminates laterally, represents the pleura of the cephalic somites.