Fourteen somites having thus been distinguished in the cephalothorax, and six being obvious in the abdomen, it is clear that there is a somite for every pair of appendages. And, if we suppose the carapace divided into segments answering to these sterna, the whole body will be made up of twenty somites, each having a pair of appendages. As the carapace, however, is not actually divided into terga in correspondence with the sterna which it covers, all we can safely conclude from the anatomical facts is that it represents the tergal region of the somites, not that it is formed by the coalescence of primarily distinct terga. In the head, and in the greater part of the thorax, the somites are, as it were, run together, but the last thoracic somite is partly free and to a slight extent moveable, while the abdominal somites are all free, and moveably articulated together. At the anterior end of the body, and, apparently, from the antennary somite, the tergal region gives rise to the rostrum, which projects between and beyond the eyes. At the opposite extremity, the telson is a corresponding median outgrowth of the last somite, which has become moveably articulated therewith. The narrowing of the sternal moieties of the anterior thoracic somites, {162} together with the sudden widening of the same parts in the posterior cephalic somites, gives rise to the lateral depression (fig. 39, _cf_) in which the scaphognathite lies. The limit thus indicated corresponds with that marked by the cervical groove upon the surface of the carapace, and separates the head from the thorax. The three pair of maxillipedes (_7, 8, 9_), the forceps (_10_), the ambulatory {163} limbs (_11–14_), and the eight somites of which they are the appendages (_VII–XIV_), lie behind this boundary and belong to the thorax. The two pairs of maxillæ (_5, 6_) the mandibles (_4_), the antennæ (_3_), the antennules (_2_), the eyestalks (_1_), and the six somites to which they are attached (_I–VI_), lie in front of the boundary and compose the head. [Illustration: FIG. 43.—_Astacus fluviatilis._—Longitudinal section of the anterior part of the cephalothorax (× 3). _I–IX_, sterna of first nine cephalothoracic somites; _1_, eyestalk; _2_, basal joint of antennule; _3_, basal joint of antenna; _4_, mandible; _a_, inner division of the masticatory surface of the mandible; _a′_, apophysis of the mandible for muscular attachment; _cp_, free edge of carapace; _e_, endosternite; _enpl_, endopleurite; _epm_, epimeral plate; _l_, labrum; _m_, muscular fibres connecting epimera with interior of carapace; _mt_, metastoma; _pcp_, procephalic process.]