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. The food, already torn and crushed by the jaws, is passed through the gullet into the cardiac sac, and there reduced to a still more pulpy state by the gastric mill. By degrees, such parts as are sufficiently fluid are drained off into the intestine, through the pyloric strainer, while the coarser parts of the useless matters are probably rejected by the mouth, as a hawk or an owl rejects his casts. There is reason to believe, though it is not certainly known, that fluids from the intestine mix with the food while it is undergoing trituration, and effect the transformation of the starchy and the insoluble protein compounds into a soluble state. At any rate, as soon as the strained-off fluid passes into the mid-gut it must be mixed with the secretion of the liver, the action of which is probably {68} similar to that of the pancreatic juice of the higher animals. [Illustration: FIG. 14.—_Astacus fluviatilis._—The corpuscles of the blood (highly magnified). _1–8_ show the changes undergone by a single corpuscle during a quarter of an hour; _9_ and _10_ are corpuscles killed by magenta, and having the nucleus deeply stained by the colouring matter. _n_, nucleus.]