We have seen that the body of the crayfish thus composed is obviously separable into three regions—the _cephalon_ or head, the _thorax_, and the _abdomen_. The latter is at once distinguished by the size and the mobility of its segments: while the thoracic region is marked off from that of the head, outwardly, only by the cervical groove. But, when the carapace is removed, the lateral depression already mentioned, in which the {142} scaphognathite lies, clearly indicates the natural boundary between the head and the thorax. It has further been observed that there are, in all, twenty pairs of appendages, the six hindermost of which are attached to the abdomen. If the other fourteen pairs are carefully removed, it will be found that the six anterior belong to the head, and the eight posterior to the thorax. The abdominal region may now be studied in further detail. Each of its seven movable segments, except the telson, represents a sort of morphological unit, the repetition of which makes up the whole fabric of the body. [Illustration: FIG. 36.—_Astacus fluviatilis._—A transverse section through the nineteenth (fifth abdominal) somite (× 2). _e.m._, extensor muscles; _f.m._, flexor muscles; _gn._ 12, the fifth abdominal ganglion; _h.g._, hind-gut; _i.a.a._, inferior abdominal artery; _s.a.a._, superior abdominal artery; _pl. XIX_, pleura of the somite; _st. XIX_, its sternum; _t. XIX_, its tergum; _ep. XIX_, its epimera; _19_, its appendages.] If the abdomen is divided transversely between the {143} fourth and fifth, and the fifth and sixth segments, the fifth will be isolated, and can be studied apart. It constitutes what is called a _metamere_; in which are distinguishable a central part termed the _somite_, and two _appendages_ (fig. 36). In the exoskeleton of the somites of the abdomen several regions have already been distinguished; and although they constitute one continuous whole, it will be convenient to speak of the _sternum_ (fig. 36, _st. XIX_), the _tergum_ (_t. XIX_), and, the _pleura_ (_pl. XIX_), as if they were separate parts, and to distinguish that portion of the sternal region, which lies between the articulation of the appendage and the pleuron, on each side, as the _epimeron_ (_ep. XIX_). Adopting this nomenclature, it may be said of the fifth somite of the abdomen, that it consists of a segment of the exoskeleton, divisible into tergum, pleura, epimera, and sternum, with which two appendages are articulated; that it contains a double ganglion (_gn._ 12), a section of the flexor (_fm_) and extensor (_em_) muscles, and of the alimentary (_hg_) and vascular (_s.a.a_, _i.a.a_) systems.