(1011). In the highest Crustacea, as the Decapoda, in which legs of an ambulatory character become such important locomotive agents, it is principally to the origins of these legs that we find the breathing apparatus appended; and their active motion will consequently powerfully contribute to the complete aeration of the blood. But let us first examine the structure of the branchiae themselves in this highly-organized division, and subsequently we will speak of their arrangement and connexions.

(1012). In the Lobster, and many other Macroura, the branchiae (fig. 204, m m), are pyramidal tufts, consisting of a central stem covered over with vascular filaments disposed perpendicularly to its axis, in such a manner that each of these organs, when detached, resembles in some degree a small brash: on cutting the stem across, it is found to enclose an artery and a vein, from which innumerable branches are given off to the horizontal filaments; so that the latter constitute a respiratory surface of great extent, which is most freely exposed to the surrounding medium.

(1013). In the Crabs and Anomoura the structure of the branchiae is somewhat different; for in these divisions the cylindrical filaments are replaced by broad lamellae laid one above the other; but in every other respect the arrangement is the same.

A side view of the arterial system in the Lobster.

Fig. 201. A side view of the arterial system in the Lobster. Nearly one half of the body is removed to show the deeper-seated vessels: a, the stomach, which has one half removed, so as to show the cavity of that viscus; 6 6 6, the intestine, which is one straight tube; c c c, the lobes of the liver of the left side, the right being removed; d, hepatic duct, entering the intestine; e, the heart; fff, three orifices, guarded by valves, whereby the venous system communicates with the heart; g, the anterior or antennal arteries; h, the posterior or superior caudal artery; i, the hepatic artery; k, a large artery going down from the posterior end of the heart, principally supplying the feet and gills (sternal artery); l is a small artery passing back along the under surface of the tail, and lost in the muscles of that part (inferior caudal artery); m, the trunk of the artery k bent forwards along the fore part of the thorax, giving off branches on each side to the feet and gills; nnn, the branches going to the feet; ooo, the branchial arteries.

(1014). The respiratory organs above mentioned are lodged in two extensive cavities, or branchial chambers, placed upon the sides of the body, covered by the broad shield of the cephalothorax (fig. 203), and lined by a membrane which is reflected upon the root of each branchia, so as to become continuous with the delicate layer that invests every filament or vascular lamella that enters into its composition.

(1015). The branchial chambers are in free communication with the external medium by means of two large apertures, through one of which the water enters, while it as constantly flows out through the other. The afferent canal is generally a wide slit that allows the water freely to penetrate to the interior of the branchial cavity; but the passage whereby the respired fluid escapes after passing over the branchiae is provided with a valvular apparatus so disposed as to produce a continual current in the water contained in the chamber, and thus, by ensuring its perpetual agitation, effectually to provide for its constant renewal. The mechanism is as follows: - The aperture by which the water issues is in the neighbourhood of the mouth, and is closed by a broad semimembranous plate (flabellum) derived from the root of the second pair of foot-jaws; so that every motion of these foot-jaws impresses a corresponding movement upon the valvelike flabellum, and in this manner urges on the passage of the water out of the cavity in which the branchiae are lodged.

(1016). But there are other means whereby the action of the limbs is made to assist in the perfection of the respiratory process. Thus, in the Lobster, the third pair of foot-jaws, and each pair of ambulatory legs, except the last, supports a flabelliform plate (fig. 204, n), the movements of which must likewise keep the fluid respired in a state of agitation, and moreover, by gently squeezing and compressing the respiratory tufts, powerfully contribute to the perfect renovation of the water in contact with the surfaces of the branchiae.

(1017). In the Crab genera the arrangement is slightly modified; for here there are three flabella, derived exclusively from the roots of the footjaws (fig. 202, b, c, d); of these, two are imbedded among the branchiae; while the third, as represented in the figure, extends in a cres-centic form over the external surface of the whole series of those organs. The end answered in this case is obviously the same as that accomplished in the Lobster, in a different and perhaps more efficient manner.

Respiratory organs of the Crab: b, c, d, foot jaws, from the bases of which the flabella are derived; f, g, h, k, I, ambulatory legs.

Fig. 202. Respiratory organs of the Crab: b, c, d, foot-jaws, from the bases of which the flabella are derived; f, g, h, k, I, ambulatory legs.

(1018). In the lowest Crustacea the heart is a long dorsal vessel, not very dissimilar in form and disposition from that of insects, but, of course, giving off arteries for the distribution of the blood, and receiving veins through which the blood, having accomplished its circuit, is returned.

(1019). In the Deca-poda the organ becomes more centralized; and in the Lobster (fig. 201, e) the heart is found to be an oval viscus, situated in the mesial line of the body, beneath the posterior part of the cephalo-thorax; it is composed of strong muscular bands, and contains a single cavity of considerable size. The contractions of this heart may readily be witnessed by raising the superjacent shell in the living animal.