Behind these cavities, the peritoneum is disposed so as to form several compartments : one, which commences at the cranial cartilage, extends downwards as far as the middle of the branchi, and contains the cesophagus, the inferior salivary glands, the crop, and anterior aorta : in front of this, but commencing a little lower down, is a second, which includes the liver and ink-bag. These two cavities are sur rounded by a common muscular tunic, of which we have already spoken, and the lower part, which resembles a diaphragm, is per forated by the gullet, the aorta, and the two biliary ducts, each of which has a distinct aperture. The receptacle which contains the gizzard is situated immediately beneath the cesophageal sac ; that in which the spiral py loric appendage is lodged lies immediately behind the left compartment of the pericar dium. The intestine is principally contained in a serous cavity behind the right division of the pericardium ; and the bottom of the sac is occupied by the cavity containing the organs of generation.
The digestive organs in the Tetrabranchiate Cephalopods would appear to differ in a less degree than other parts of their organization from the structures observable in the higher order : in the Nautilus they present the following con formation.
The pharynx (f, 217) or commence ment of the gullet, has numerous longitudinal rugm internally, and is evidently capable of con siderable dilatation. The cesophagus, after having passed beneath the brain, or commissure of the optic ganglions, dilate into a capacious pouch or crop (k,.fig. 219) of a pyriform shape, two inches and three lines in length, and an inch in diameter at the broadest part. From the bottom of this crop is continued a contracted canal (4fig.219,) of about three lines in diame ter, and half an inch in length, which enters the upper part of an oval gizzard (m, fig. 219) situated at the bottom of the pallial sac. Close to where this tube enters, the intestine (n, fig.219) is continued from the gizzard, and after a course of a few lines communicates with a small round laminated pouch or ap pendage (p, fig. 219) analogous to the spiral ccecum of the Cuttlefish, into which the binary secretion is poured : from thence the intestine is continued, twice bent upon itself, but with out varying materially in its dimensions, to its termination (0, fig. 219). In this course it first ascends for about an inch and a half, then makes a sudden bend down to the bottom of the sac, and returns as suddenly upon itself, passing close to the pericardium, and terminat ing between the roots of the branchiw.
The alimentary canal is every where con nected to the parietes of the abdomen by numerous filaments ; the only trace of a me sentery exists between the two last portions of the intestine, which are connected together by rnembranes including the ramifications of an artery and vein.*
The longitudinal rugm, into which the lining membrane of the cesophagus is thrown, disappear at its entrance into the crop. The muscular coat of the crop con sists of an exterior layer of close-set circu lar fibres and an inner layer of more scattered longitudinal ones. The lining c, membrane is thin but tough, with a smooth surface : when the cavity is empty, it is probably thrown into longitudinal folds by the action of the circular fibres.
In the canal which leads to the gizzard, the lining membrane puts on a villous appearance and is disposed in distinct close-set longitudinal rugm.
The gizzard is girt by two broad radiate muscles, of the thickness of two lines, arising from opposite tendons : it is lined by a thick cuticular membrane, delicately furrowed and adapted to numerous fine ridges which tra verse longitudinally the whole interior of the cavity. This, as is commonly found in gizzards, was detached from part of the parietes and adhered very slightly to the remainder.
The pyloric orifice is close to the cardiac, and is guarded by a valve, to prevent a too ready egress of matter from the gizzard.* The globular cavity (p, fig. 219) which communicates with the intestine at a little dis tance from the pylorus, is occupied with broad parallel laminm, which are puckered trans versely, so as to increase their surface for vas cular ramifications ; their texture under the lens is follicular and evidently fitted to secrete. The bile enters this cavity at the extremity furthest from the intestine by a duct large enough to admit a common probe. The two laminw on each side the entrance of the duct increase in breadth as they approach the in testine, and are continued in a curved form along that canal, being gradually lost in its inner membrane, the lamina next the gizzard is peculiarly enlarged, so as evidently to pre sent an obstacle to the regurgitation of bile towards the gizzard. The inner surface of the rest of the intestinal canal presents a few lon gitudinal ruge, with slightly marked transverse puckerings.
In the Dibranchiate Cephalopods the gul let, in consequence of the position of the stomach near the lower part of the visceral sac, is of great length (a, a, fig. 221), but varies in this respect according to the form of the animal. We have seen that in the Nautilus it is dilated into a pyriform crop; a similar dilatation occurs in the genus Octo pus ; but its position is reversed, the larger end of the sac being uppermost, and probably as the result of the habitually reversed position of the animal with the head downwards, the crop is extended into a large cul-de-sac above the part where the cesophagus opens into it (b, fig. 220). From this part the crop gradually contracts to its termination.