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.
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.
The mixture thus produced, which answers to the chyle of the higher animals, passes along the intestine, and the greater part of it, transuding through the walls of the alimentary canal, enters the blood, while the rest accumulates as dark coloured fæces in the hind gut, and is eventually passed out of the body by the vent. The fæcal matters are small in amount, and the strainer is so efficient that they rarely contain solid particles of sensible size. Sometimes, however, there are a good many minute fragments of vegetable tissue.
The blood of which the nutritive elements of the food {69} have thus become integral parts, is a clear fluid, either colourless, or of a pale neutral tint or reddish hue, which, to the naked eye, appears like so much water. But if subjected to microscopic examination, it is found to contain innumerable pale, solid particles, or corpuscles, which, when examined fresh, undergo constant changes of form (fig. [14]). In fact, they correspond very closely with the colourless corpuscles which exist in our own blood; and, in its general characters, the crayfish’s blood is such as ours would be if it were somewhat diluted and deprived of its red corpuscles. In other words, it resembles our lymph more than it does our blood. Left to itself it soon coagulates, giving rise to a pretty firm clot.
The sinuses, or cavities in which the greater part of the blood is contained, are disposed very irregularly in the intervals between the internal organs. But there is one of especially large size on the ventral or sternal side of the thorax (fig. [15], sc), into which all the blood in the body sooner or later makes its way. From this sternal sinus passages (av) lead to the gills, and from these again six canals (bcv), pass up on the inner side of the inner wall of each branchial chamber to a cavity situated in the dorsal region of the thorax, termed the pericardium (p), into which they open.
FIG. 15.—Astacus fluviatilis.—A diagrammatic transverse section of the thorax through the twelfth somite, showing the course of the circulation of the blood (× 3). arb. 12, the anterior or lower, and arb′. 12, the posterior or upper arthrobranchia of the twelfth somite; av, afferent branchial vessel; bcv, branchio-cardiac vein; bg, branchiostegite; em, extensor muscles of abdomen; ep, epimeral wall of thoracic cavity; ev, efferent branchial vessel; fm, flexor muscles of abdomen; fp, floor of pericardium; gn. 6, fifth thoracic ganglion; h, heart; hg, hind-gut; iaa, inferior abdominal artery, in cross section; la, lateral valvular apertures of heart; lr, liver; mp, indicates the position of the mesophragm by which the sternal canal is bounded laterally; p, pericardial sinus; pdb. 12, podobranchia, and plb. 12, pleurobranchia of the twelfth somite; sa, sternal artery; saa, superior abdominal artery; sc, sternal canal; t, testis; XII., sternum of twelfth somite. The arrows indicate the direction of the blood flow.
The blood of the crayfish is kept in a state of constant circulating motion by a pumping and distributing machinery, composed of the heart and of the arteries, with {71} their larger and smaller branches, which proceed from it and ramify through the body, to terminate eventually in the blood sinuses, which represent the veins of the higher animals.
When the carapace is removed from the middle of the region which lies behind the cervical groove, that is, when the dorsal or tergal wall of the thorax is taken away, a spacious chamber is laid open which is full of blood. This is the cavity already mentioned as the pericardium (fig. [15], p), though, as it differs in some respects from that which is so named in the higher animals, it will be better to term it the pericardial sinus.