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round the stomach to the antennæ (aa). Behind these, yet two other arteries are given off from the under side of the heart, and supply the liver (ha). All these arteries branch out and eventually terminate in fire, so-called capillary, ramifications.

In the dorsal wall of the heart two small oval apertures are visible, provided with valvular lips (fig. 16, sa), which open inwards, or towards the internal cavity of the heart. There is a similar aperture in each of the lateral faces of the heart (la), and two others in its inferior face (ia), making six in all. These apertures readily admit fluid into the heart, but oppose its exit. On the other hand, at the origins of the arteries, there are small valvular folds, directed in such a manner as to permit the exit of fluid from the heart, while they prevent its entrance.

The walls of the heart are muscular, and, during life, they contract at intervals with a regular rhythm, in such a manner as to diminish the capacity of the internal cavity of the organ. The result is, that the blood which it contains is driven into the arteries, and necessarily forces into their smaller ramifications an equivalent amount of the blood which they already contained; whence, in the long run, the same amount of blood passes out of the ultimate capillaries into the blood sinuses. From the disposition of the blood sinuses, the impulse thus given to the blood which they contain is finally conveyed to the blood in the branchia, and a proportional quantity of that

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 (× 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

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round the stomach to the antennæ (aa). Behind these, yet two other arteries are given off from the un ler si le of the heart, and supply the liver (ha). All these arteries branch out and eventually terminate in file, so-called capillary, ramifications.

In the dorsal wall of the heart two small oval aper tures are visible, provided with valvular lips (fig. 16, sa), which open inwards, or towards the internal cavity of the heart. There is a similar aperture in each of the lateral faces of the heart (la), and two others in its inferior face (ia), making six in all. These apertures readily admit fluid into the heart, but oppose its exit. On the other hand, at the origins of the arteries, there are small valvular folds, directed in such a manner as to permit the exit of fluid from the heart, while they prevent

its entrance.

The walls of the heart are muscular, and, during life, they contract at intervals with a regular rhythm, in such a manner as to diminish the capacity of the internal cavity of the organ. The result is, that the blood which it contains is driven into the arteries, and necessarily forces into their smaller ramifications an equivalent amount of the blood which they already contained; whence, in the long run, the same amount of blood passes out of the ultimate capillaries into the blood sinuses. From the disposition of the blood sinuses, the impulse thus given to the blood which they contain is finally conveyed to the blood in the branchiæ, and a proportional quantity of that

blood leaves the branchiæ and passes into the sinuses which connect them with the pericardial sinus (fig. 15, bcv), and thence into that cavity. At the end of the contraction, or systole, of the heart, its volume is of course diminished by the volume of the blood forced out, and the space between the walls of the heart and those of the pericardial sinus is increased to the same extent. This space, however, is at once occupied by the blood from the branchiæ, and perhaps by some blood which has not passed through the branchiæ, though this is doubtful. When the systole is over, the diastole follows; that it to say, the elasticity of the walls of the heart and that of the various parts which connect it with the walls of the pericardium, bring it back to its former size, and the blood in the pericardial sinus flows into its cavity by the six apertures. With a new systole the same process is repeated, and thus the blood is driven in a circular course through all parts of the body.

It will be observed that the branchiæ are placed in the course of the current of blood which is returning to the heart; which is the exact contrary of what happens in fishes, in which the blood is sent from the heart to the branchiæ, on its way to the body. It follows, from this arrangement, that the blood which goes to the branchia is blood in which the quantity of oxygen has undergone a diminution, and that of carbonic acid an increase, as compared with the blood in the heart itself. For the

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activity of all the organs, and especially of the muscles, is inseparably connected with the absorption of oxygen and the evolution of carbonic acid; and the only source from which the one can be derived, and the only receptacle into which the other can be poured, is the blood which bathes and permeates the whole fabric to which it is distributed by the arteries.

The blood, therefore, which reaches the branchiæ has lost oxygen and gained carbonic acid; and these organs constitute the apparatus for the elimination of the injurious gas from the economy on the one hand, and, on the other, for the taking in of a new supply of the needful "vital air," as the old chemists called it. It is thus that the branchiæ subserve the respiratory function.

The crayfish has eighteen perfect and two rudimentary branchiæ in each branchial chamber, the boundaries of which have been already described.

Of the eighteen perfect branchiæ, six (podobranchiæ) are attached to the basal joints of the thoracic limbs, from the last but one to the second (second maxillipede) inclusively (fig. 4, p. 26, pdb, and fig. 17, A, B); and eleven (arthrobranchia) are fixed to the flexible interarticular membranes, which connect these basal joints with the parts of the thorax to which they are articulated (fig. 4, arb, arb', fig. 17, C). Of these eleven branchiæ, two are attached to the interarticular membranes of all the ambulatory legs but the last, (=6) and to those of the pincers and of the external maxillipedes, (=4) and one to that of the