similar to that of the pancreatic juice of the higher animals.

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

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.

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

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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.

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

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; bev, 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.

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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.

The heart (fig. 15, h), lies in the midst of this sinus. It is a thick muscular body (fig. 16), with an irregularly hexagonal contour when viewed from above, one angle of the hexagon being anterior and another posterior. The lateral angles of the hexagon are connected by bands of fibrous tissue (ac) with the walls of the pericardial sinus. Otherwise, the heart is free, except in so far as it is kept in place by the arteries which leave it and traverse the walls of the pericardium. One of these arteries (figs. 5, 12, and 16, saa), starting from the hinder part of the heart, of which it is a sort of continuation, runs along the middle line of the abdomen above the intestine, to which it gives off many branches. A second large artery starts from a dilatation, which is common to it with the foregoing, but passing directly downwards (figs. 12 and 15, sa, and fig. 16, st. a), either on the right or on the left side of the intestine,

traverses the nervous cord (figs. 12 and 15), and divides into an anterior (fig. 12, sa) and a posterior (iaa) branch, both of which run beneath and parallel with that cord.

FIG 16.-Astacus fluviatilis.-The heart (x 4). A, from above; B, from below; C, from the left side. aa, antennary artery; ac, alæ cordis, or fibrous bands connecting the heart with the walls of the pericardial sinus; b, bulbous dilatation at the origin of the sternal artery; ha. hepatic artery; la, lateral valvular apertures; oa, ophthalmic artery; s.a, superior valvular apertures; s.a.a, superior abdominal artery; st.a, sternal artery, in B cut off close to its origin.

A third artery runs, from the front part of the heart, forwards in the middle line, over the stomach, to the eyes and fore part of the head (figs. 5, 12, and 16, oa); and two others diverge one on each side of this, and sweep