The systemic ventricle (o) is situated in the mesial plane between the bifurcation of the vena cava above, and the ovary or testis below. In the Octopus and Eledone it presents a glo bular form, rather extended tranversely, and with the branchial sinus entering at its superior and lateral aspects. In the Loligo and the azychoteuthis (fig. 227) it is lozenge-shaped, with the long axis in the axis of the body ; giving off the two aorta3 (c, d) by the anterior and posterior angles, and receiving the bran chial veins (a,a,) at the lateral angles. In the Sepia, (o, fig. 225,) Sepioteuthis, and Rossia, the systemic ventricle is a fusiform body, bent upon itself at right angles. About one-half on the right side lies in the axis of the body, the remainder extends transversely to the left side ; the extremity of this part receives the left bran chial vein, the other extremity gives off the an terior aorta (g, fig, 225). The bulb of the posterior and generally the larger aorta (p, fig. 225) is continued from the middle of the transverse portion ; the right -branchial vein enters the middle of the right side of the lon gitudinal portion of the ventricle.
In all the Dibranchiata the parietes of the systemic heart, though thin, are firmer and more muscular than those of the branchial hearts; and its cavity is generally about three times greater than that of either of the others : its inner surface shows the regular interlacement and decussation of the columme carnew, none of which, however, project into the cavity. The termination of each branchial vein is defended by a pair of membranous semi lunar valves (b, fig. 227). The origin of the lesser aorta (p), arising from the anterior part of the ventricle, is defended by a single valve (e,fig.227); that of the great aorta, (gi,fig.
226,) which, though posterior in its origin, is de stined to supply the head and anterior parts of the body, is generally provided with a mus cular bulb, as in the Nautilus. In the Octopus it is defended, according to Cuvier, by two semilunar valves ; but in the Calamary and Onychoteuthis by a single valve (f fin.. 227).
. b In the Octopus there is also a third small artery (r, fig. 225) given off directly from the ventricle, which is distributed to the generative organs, and presents considerable periodical variations of size in relation to the functions of those parts. In the same genus the small aorta, which arises from the anterior part of the ventricle, first gives off two long and slender branches (s, s, fig. 226), which are distributed to the venous follicles, whose arterial vascularity we have before mentioned. The trunk then di vides into two arteries, of which the largest (t) ascends in front of the vena cava to be distri buted to the mantle ; the other supplies the folded intestine and surrounding peritoneum. The large aorta first passes backwards and to the right between the layers of peritoneum which separate the intestinal sac from that of the pyloric appendage and that of the stomach; winds round the latter, and passes, by a proper opening, to the right of the cardia through the muscular septum, and into the cavity behind the liver, and ascends on the right side of the dilated cesophagus to the cartilaginous cranium. Here, after distributing branches to the sur rounding parts, it bifurcates and completely encircles the gullet; and from this vascular ring, which is strikingly analogous to the bran chial arches in Vertebrata, the head and all its complex radiating appendages derive their nu triment.