[Illustration: FIG. 12.—_Astacus fluviatilis._—A dissection of a male specimen from the right side (nat. size). _a_, anus; _aa_, antennary artery, cut short; _ag_, anterior gastric muscles, the right cut away to its insertion; _bd_, aperture of right bile duct; _cm_, constrictor muscles of stomach; _cæ_, cæcum; _cpm_, right cardio-pyloric muscle; _cs_, cardiac portion of stomach; _cm_, extensor muscles of abdomen; _fm_, flexor muscles of abdomen; _ga_, gastric artery; _gn._ 1, supraœsophageal ganglion; _gn._ 2, sub-œsophageal ganglion; _gn._ 13, last abdominal ganglion; _h_, heart; _ha_, hepatic artery; _hg_, hind-gut; _iaa_, inferior abdominal artery; _la_, right lateral aperture of heart; _lr_, left liver; _mg_, mid-gut; _oa_, ophthalmic artery; _œ_, œsophagus; _pg_, posterior gastric muscles, the right cut away to its insertion; _ps_, pyloric portion of stomach; _sa_, sternal artery; _saa_, superior abdominal artery; _t_ (to the left), telson; _t_ (near the heart), testis; _vd_, left vas deferens; _vd′_, aperture of left vas deferens; _2_, right antennule; _4_, left mandible; _9_, left external maxillipede; _10_, left forceps; _15_, first, _16_, second, and _20_, sixth abdominal appendages of the left side.] If the treatment to which the food is subjected in the alimentary apparatus were of a purely mechanical nature, there would be nothing more to describe in this part of the crayfish’s mechanism. But, in order that the nutritive matters may be turned to account, and undergo the chemical metamorphoses, which eventually change them into substances of a totally different character, they must pass out of the alimentary canal into the blood. And they can do this only by making their way through the walls of the alimentary canal; to which end they must either be in a state of extremely fine division, or they must be reduced to the fluid condition. In the case of the fatty matters, minute subdivision may suffice; but the amylaceous substances and the insoluble protein compounds, such as the fibrin of flesh, must be brought into a state of solution. Therefore some substances must be poured into the alimentary canal, which, when mixed with the crushed food, will play the part of a chemical agent, dissolving out the insoluble proteids, changing the amyloids into soluble sugar, and converting all the proteids into those diffusible forms of protein matter, which are known as _peptones_. The details of the processes here indicated, which may be included under the general name of _digestion_, have only quite recently been carefully investigated in the crayfish; and we have probably still much to learn about {64} them; but what has been made out is very interesting, and proves that considerable differences exist between crayfishes and the higher animals in this respect.