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. [Illustration: 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.