(1550). The Cephalopoda breathe by means of branchiae, and possess a complex and elaborate circulatory system, organized upon very extraordinary principles, to the consideration of which we now invite the attention of the reader.

(1551). The branchiae (fig. 290, g g) in all the genera now known to exist, with the exception of the Nautilus, are two in number, one situated on each side of the body; but in the Nautilus Pompilius there are four branchial organs, two on each side; and hence Professor Owen has divided the class into two great orders, under the names of Dibran-chiata and Tetrabranchiata, - the former embracing all the ordinary genera, while the latter is, as far as we know, only represented in modern times by the Pearly Nautilus, depicted in a preceding figure.

(1552). In both the Dibranchiate and Tetrabranchiate orders, each branchia consists of a broad central stem, to which is appended a series of vascular lamellae, seen in the figure given below (fig. 290, g): by this arrangement a very extensive surface is obtained, over which the blood is diffused for the purpose of respiration. The respiratory apparatus is lodged within the visceral sac, but separated from the other viscera by a membranous septum (fig. 289, t); so that a distinct chamber is formed to contain the branchia), into which the water is freely admitted, - the surrounding element being alternately drawn into the branchial cavity, by the action of its muscular walls, through a valvular aperture provided for the purpose, and again expelled in powerful streams through the orifice of the funnel. Such, indeed, is the force with which the water is ejaculated through the funnel, that it not only serves to expel from the body excrementitious matter derived from the termination of the rectum (fig. 290, s), which opens into the respiratory cavity, but becomes one of the ordinary agents in locomotion. This mode of progression, although in fact common to most of the Cephalopod tribes, is remarkably exemplified in the Argonaut, which, instead of navigating the surface of the sea, as has been already stated, simply darts itself from place to place by sudden and oft-repeated jets thus violently spouted forth, while with its arms stretched out and closely approximated, and its vela tightly expanded over the outward surface of its delicate shell, it shoots backwards like an arrow through the water.

Anatomy of the Cuttle fish (after Hunter): a a, section of the mantle.

Fig. 290. Anatomy of the Cuttle-fish (after Hunter): a a, section of the mantle; b, c, venae cavae; d, spongoid appendages of ditto; e e, branchial hearts; m m, their lateral appendages; ff, ligaments of branchiae; g, branchial organs; h, branchial vein; i i, systemic auricles; k, systemic ventricle; o, the stomach; p, q, the ovaria.

(1553). Separated from the chamber in which the branchiae are lodged, by the membranous partition already mentioned (fig. 290, t), and likewise distinct from the peritoneum containing the viscera, is a considerable cavity, divided by a membranous partition into two compartments, wherein are placed the great trunks of the venous system (d d.) These chambers, named by Cuvier* the "great venous cavities," are very remarkable, inasmuch as, although they contain the vence cavae, which here present a truly anomalous structure, they are lined with a mucous membrane derived from the branchial chamber, with which they are in free communication, and from whence the external element has free admission to their interior.

(1554). It is in this "great venous cavity" called by Professor Owen the "pericardium," that, in the Pearly Nautilus, the siphon which traverses the partitions of its camerated shell (fig. 284) terminates; and the reader will now perceive by what mechanism water received from the branchial chamber may, in that animal, be injected into its partitioned shell for the purpose already referred to (§ 1517).

(1555). In the "great venous cavity," or "pericardium" thus formed, are lodged the principal venous trunks (fig. 290, d d), into which the blood derived from all parts of the body is brought by capacious vessels (b, c c) that may be called the vence cavae. The great central receptacles of the venous blood (d d), whilst they are contained in the pericardium (or, rather, project into its interior, being partially covered with the mucous membrane that lines its walls), are enveloped by a mass of spongy appendages of a most remarkable and peculiar description. These spongy masses are of a yellow colour, and, when squeezed, they give out an opake yellowish mucosity 1; but the most interesting circumstance connected with these bodies is, that they communicate by large and patulous apertures with the interior of the veins to which they are adherent. The short canals derived from these apertures are themselves pierced by very numerous orifices, and so on successively, until each of the spongy bodies referred to is permeated internally by a multitude of short vessels leading one into another, and ultimately into the vein itself.

Cuvier supposes that, seeing it is impossible that these vessels should not be filled with blood, they might themselves be considered as veins; but then their extent, when compared with the very small arteries of the spongy bodies, forbids us to believe that they have no other office than that of bringing back into the general current of the venous circulation blood derived from these arterial ramifications. He suggests, therefore, that they more probably form diverticula in which the venous blood may become diffused, in order to receive, through the intervention of their spongy walls, the influence of the surrounding medium; so that in this way they may be subservient to respiration; or else it is possible that the orifices in the veins are the openings of excretory canals derived from these appendages, through which they may pour into the vein some substance derived from the water in which they float. Lastly, it is conjectured that they may be emunc-tories, through which some principle separated from the blood is discharged from the body through the pores upon their surface - a supposition rendered more probable, seeing the abundant mucous secretion that may be extracted from them by pressure. "However this may be," observes Cuvier, "it is certain that the communication between these bodies and the exterior is very open; for on blowing into or injecting the vein, the air or injection passes very readily into the cavity that the vein traverses; and, on the other hand, on inflating the cavity from the branchial chamber, it often happens that the vein becomes filled with air".