The gullet is lined by a firm coat which resembles thin parchment. At the margins of the mouth, this strong lining is easily seen to be continuous with the cuticular exoskeleton; while, at the cardiac orifice, it spreads out and forms the inner or cuticular wall of the whole gastric cavity, as far as the pylorus, where it ends in certain valvular projections. The chitinous cuticle which forms the outermost layer of the integument is thus, as it were, turned in, to constitute the innermost layer of the walls of the stomach; and it confers upon them so great an amount of stiffness that they do not collapse when the organ is removed from the body. Furthermore, just as the cuticle of the integument is calcified to form the hard parts of the exoskeleton, so is the cuticle of the stomach calcified, or otherwise hardened, to give rise, in the first {53} place, to the very remarkable and complicated apparatus which has already been spoken of, as a sort of _gastric mill_ or _food-crusher_; and, secondly, to a _filter_ or _strainer_, whereby the nutritive juices are separated from the innutritious hard parts of the food and passed on into the intestine. [Illustration: FIG. 9.—_Astacus fluviatilis._—A, the stomach with its outer coat removed, seen from the left side; B, the same viewed from the front, after removal of the anterior wall; C, the ossicles of the gastric mill separated from one another; D, the prepyloric ossicle and median tooth, seen from the right side; E, transverse section of the pyloric region along the line _xy_ in A (all × 2). _c_, cardiac ossicle; _cpv_, cardio-pyloric valve; _lp_, lateral pouch; _lt_, lateral tooth, seen through the wall of the stomach in A; _mg_, mid-gut; _mt_, median tooth, seen through the wall of the stomach in A; _œs_, œsophagus; _p_, pyloric ossicle; _pc_, pterocardiac ossicle; _pp_, prepyloric ossicle; _uc_, uro-cardiac process; _t_, convexities on the free surface of its hinder end; _v_^1, median pyloric valve; _zc_, zygocardiac ossicle.] {54}