As I have already mentioned, when dealing with the physiology of the crayfish, the entire body of the animal, when reduced to its simplest morphological expression, may be represented as a cylinder, closed at each end, except so far as it is perforated by the alimentary apertures (fig. [6]); or we may say that it is a tube, inclosing another tube, the edges of the two being continuous at their extremities. The outer tube has a chitinous outer coat or cuticle, which is continued on to the inner face of the inner tube. Neglecting this for the present, the outermost part of the wall of the outer tube, which answers to the epidermis of the higher animals, and the innermost part of the wall of the inner tube, which is an epithelium, are formed by a layer of nucleated cells. A continuous layer of cells, therefore, is everywhere to {141} be found on both the external and the internal free surfaces of the body. So far as these cells belong to the proper external wall of the body, they constitute the ectoderm, and so far as they belong to its proper internal wall, they compose the endoderm. Between these two layers of nucleated cells lie all the other parts of the body, composed of connective tissue, muscles, vessels, and nerves; and all these (with the exception of the ganglionic chain, which we shall see properly belongs to the ectoderm) may be regarded as a single thick stratum, which, as it lies between the ectoderm and the endoderm, is called the mesoderm.
If the intestine were closed posteriorly instead of opening by the vent, the crayfish would virtually be an elongated sac, with one opening, the mouth, affording an entrance into the alimentary cavity: and, round this cavity, the three layers just referred to—endoderm, mesoderm, and ectoderm—would be disposed concentrically.
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.
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.