[Illustration: FIG. 11.—_Astacus fluviatilis._—View of the roof of the stomach, the ventral wall of which, and of the mid-gut, is laid open by a longitudinal incision (× 4). On the right side (the left in the figure), the lateral tooth is cut away, as well as the floor of the lateral pouch. The letters have the same signification as in fig. 10.] The cuticular lining which gives rise to all the complicated apparatus which has just been described, must not be confounded with the proper wall of the stomach, which invests it, and to which it owes it origin, just as the cuticle of the integument is produced by the soft {61} true skin which lies beneath it. The wall of the stomach is a soft pale membrane containing variously disposed muscular fibres; and, beyond the pylorus, it is continued into the wall of the intestine. It has already been mentioned that the intestine is a slender and thin-walled tube, which passes straight through the body almost without change, except that it becomes a little wider and thicker-walled near the vent. Immediately behind the pyloric valves, its surface is quite smooth and soft (figs. 9, 10, and 12, _mg_), and its floor presents a relatively large aperture, the termination of the bile duct (fig. 12, _bd_, fig. 10, _hp_), on each side. The roof is, as it were, pushed out into a short median pouch or _cæcum_ (_cæ_). Behind this, its character suddenly changes, and six squarish elevations, covered with a chitinous cuticle, encircle the cavity of the intestine (_r_). From each of these, a longitudinal ridge, corresponding with a fold of the wall of the intestine, takes its rise, and passes, with a slight spiral twist, to its extremity (_hg_). Each of these ridges is beset with small papillæ, and the chitinous lining is continued over the whole to the vent, where it passes into the general cuticle of the integument, just as the lining of the stomach is continuous with the cuticle of the integument at the mouth. The alimentary canal may, therefore, be distinguished into a _fore_ and a _hind-gut_ (_hg_), which have a thick internal lining of cuticular membrane; and a very short _mid-gut_ (_mg_), which has no thick cuticular layer. It will be of {63} importance to recollect this distinction by-and-by, when the development of the alimentary canal is considered.