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.
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.
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.
The epithelium is made up of nucleated cells, which are particles of simple living matter, or protoplasm, in the midst of each of which is a rounded body, which is termed the nucleus. It is these cells which are the seat of the manufacturing process which results in the formation of the secretion; it is, as it were, their special business to form that secretion. To this end they are constantly being newly formed at the summits of the cæca. As they grow, they pass down towards the duct and, at the same time, separate into their interior certain special products, among which globules of yellow fatty matter are very conspicuous. When these products are fully formed, what remains of the substance of the cells dissolves away, and {67} the yellow fluid accumulating in the ducts passes into the mid-gut. The yellow colour is due to the globules of fat. In the young cells, at the summit of the cæca, these are either absent, or very small, whence the part appears colourless. But, lower down, small yellow granules appear in the cells, and these become bigger and more numerous in the middle and lower parts. In fact, few glands are better fitted for the study of the manner in which secretion is effected than the crayfish’s liver.
We may now consider the alimentary machinery, the general structure of which has been explained, in action.