[Illustration: FIG. 5.—_Astacus fluviatilis._—A male specimen, with the roof of the carapace and the terga of the abdominal somites removed to show the viscera (nat. size):—_aa_, antennary artery; _ag_, anterior gastric muscles; _amm_, adductor muscles of the mandibles; _cs_, cardiac portion of the stomach; _gg_, green glands; _h_, heart; _hg_, hind gut, or large intestine; _Lr_, liver; _oa_, ophthalmic artery; _pg_, posterior gastric muscles; _saa_, superior abdominal artery; _t_, testis; _vd_, vas deferens.] Thus, when the carapace is removed in a crayfish which has been just killed, the heart is seen still pulsating. It is an organ of considerable relative size (fig. 5, _h_), which is situated immediately beneath the {29} middle region of that part of the carapace which lies behind the cervical groove; or, in other words, in the dorsal region of the thorax. In front of it, and therefore in the head, is a large rounded sac, the stomach (fig. 5, _cs_; fig. 6, _cs_, _ps_), from which a very delicate intestine (figs. 5 and 6, _hg_) passes straight back through the thorax and abdomen to the vent (fig. 6, _a_). [Illustration: FIG. 6.—_Astacus fluviatilis._—A longitudinal vertical section of the alimentary canal, with the outline of the body (nat. size):—_a_, vent; _ag_, anterior gastric muscle; _bd_, entrance of left bile duct; _cg_, cervical groove; _cæ_, cæcum; _cpv_, cardio-pyloric valve; _cs_, cardiac portion of stomach; the circular area immediately below the end of the line from _cs_ marks the position of the gastrolith of the left side; _hg_, hind-gut; _lb_, labrum; _lt_, lateral tooth of stomach; _m_, mouth; _mg_, mid-gut; _mt_, median tooth; _œ_, œsophagus; _pc_, procephalic process; _pg_, posterior gastric muscle; _ps_, pyloric portion of stomach; _r_, annular ridge, marking the commencement of the hind-gut.] In summer, there are commonly to be found at the sides of the stomach two lenticular calcareous masses, which are known as “crabs’-eyes,” or _gastroliths_, and were, in old times, valued in medicine as sovereign remedies for all sorts of disorders. These bodies (fig. 7) are smooth and flattened, or concave, on the side which is turned towards {30} the cavity of the stomach; while the opposite side, being convex and rough with irregular prominences, is something like a “brain-stone” coral. [Illustration: FIG. 7.—_Astacus fluviatilis._—A gastrolith; A, from above; B, from below; C, from one side (all × 5); D, in vertical section (× 20).]