The physiologist calls those organs, the function of which is to prepare and discharge substances of a special character, _glands_; and the matter which they elaborate is termed their _secretion_. On the one side, glands are in relation with the blood, whence they derive the materials which they convert into the substances characteristic of their secretion; on the other side, they have access, directly or indirectly, to a free surface, on to which they pour their secretion as it is formed. Of such glands, the alimentary canal of the crayfish is provided with a pair, which are not only of very large size, but are further extremely conspicuous, on account of their yellow or brown colour. These two glands (figs. 12 and 13, _lr_) are situated beneath, and on each side of, the stomach and the anterior part of the intestine, and answer in position to the glands termed liver and pancreas in the higher animals, inasmuch as they pour their secretion into the mid-gut. These glands have hitherto always been regarded as the _liver_, and the name may be retained, though their secretion appears rather to correspond with the pancreatic fluid than with the bile of the higher animals. [Illustration: FIG. 13.—_Astacus fluviatilis._—The alimentary canal and livers seen from above (nat. size). _bd_, bile-duct; _cæ_, cæcum; _cs_, cardiac portion of stomach, the line pointing to the cardiac ossicle; _hg_, hind-gut; _mg_, mid-gut; _pc_, pterocardiac ossicle; _ps_, pyloric portion of stomach, the line pointing to the pyloric ossicle; _r_, ridge separating mid-gut from hind-gut; _zc_, zygocardiac ossicle.] Each liver consists of an immense number of short tubes, or _cæca_, which are closed at one end, but open at the other into a general conduit, which is termed their _duct_. The mass of the liver is roughly divided into {66} three lobes, one anterior, one lateral, and one posterior; and each lobe has its main duct, into which all the tubes composing it open. The three ducts unite together into a wide common duct (_bd_), which opens, just behind the pyloric valves, into the floor of the mid-gut. Hence the apertures of the two _hepatic ducts_ are seen, one on each side, in this part of the alimentary canal when it is laid open from above. Every cæcum of the liver has a thin outer wall, lined internally by a layer of cells, constituting what is termed an _epithelium_; and, at the openings of the hepatic ducts, this epithelium passes into a layer of somewhat similar structure, which lines the mid-gut, and is continued through the rest of the alimentary canal, beneath the cuticula. Hence the liver may be regarded as a much divided side pouch of the mid-gut.