The endosternite (fig. [42], ens.) ascends vertically, with a slight inclination forwards, and its summit narrows and assumes the form of a pillar, with a flat, transversely elongated capital. The inner prolongation of the capital is called the mesophragm (mph.), the outer paraphragm (pph.). The mesophragms of the two endosternites of a somite usually unite by a median suture, and thus form a complete arch over the sternal canal (s.c.), which lies between the endosternites. {159}
The endopleurites (en.pl.) are also vertical plates, but they are relatively shorter, and their inner angles give off two nearly horizontal processes, one of which passes obliquely forwards (fig. [39], B, h, fig. [42], h.p.) and unites with the paraphragm of the endosternite of the somite in front, while the other, passing obliquely backwards (fig. [39], h′), becomes similarly connected with the endosternite of the somite behind.
FIG. 42.—Astacus fluviatilis.—A segment of the endophragmal system (× 3). aph, arthrophragm; arth, arthrodial or articular cavity; cxp, coxopodite of the ambulatory leg; enpl, endopleurite; ens, endosternite; epm, epimeron; hp, horizontal process of endopleurite; mph, mesophragm; pph, paraphragm; s, sternum of somite; sc, sternal canal.
The endopleurites of the last thoracic somite are rudimentary, and its endosternites are small. On the other hand, the mesophragmal processes of the endosternites of the two posterior somites of the head (fig. [39], B, c.ap), by which the endophragmal system terminates in front, are particularly strong and closely united together. They thus, with their endopleurites, form a solid partition between the stomach, which lies upon them, and the mass of {160} coalesced anterior thoracic and posterior cephalic ganglia situated beneath them. Strong processes are given off from their anterior and outer angles, which curve round the tendons of the adductor muscles of the mandibles, and give attachment to the abductors.
In front of the mouth there is no such endophragmal system as that which lies behind it. But the anterior gastric muscles are attached to two flat calcified plates, which appear to lie in the interior of the head (though they are really situated in its upper and front wall) on each side of the base of the rostrum, and are called the procephalic processes (figs. [40], [43], p.cp). Each of these plates constitutes the posterior wall of a narrow cavity which opens externally into the roof of the orbit, and has been regarded (though, as it appears to me, without sufficient reason) as an olfactory organ. I am disposed to think, though I have not been able to obtain complete evidence of the fact, that the procephalic processes are the representatives of the “procephalic lobes” which terminate the anterior end of the body in the embryo crayfish. At any rate, they occupy the same position relatively to the eyes and to the carapace; and the hidden position of these processes, in the adult, appears to arise from the extension of the carapace at the base of the rostrum over the fore part of the originally free sternal surface of the head. It has thus covered over the procephalic processes, in which the sternal wall of the body terminated; and the cavities which lie in front of them are {161} simply the interspaces left between the inferior or posterior wall of the prolongation of the carapace and the originally exposed external faces of these regions of the cephalic integument.
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.
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.