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476 further remarks on this subject until we have taken a glance at the vascular system.

According to Milne Edwards, the blood system in these animals is incomplete, as it is in all other mollusks. And in proof of this, a large sinus, or lacune, is referred to in the Octopodidæ, which is asserted to be the homologue of the abdominal or visceral chamber usually observed in the Mollusca. This sinus is situated on the dorsal region of the body, and extends nearly the whole length of the animal. It is divided by constrictions into three compartments; the anterior, or buccal, the median, or œsophageal, and the posterior, or gastric. With the exception of the anterior portion, which lies in the midst of the fleshy mass forming the base of the arms, this compound sinus is placed between the liver and the wall of the body. The posterior division communicates with the median portion by an orifice, not much wider than is sufficient to allow of the easy passage of the lower extremity of the œsophagus, or crop. It contains the gizzard, the spiral stomach or cæcum, and the hepatic ducts. The gastric organs are suspended in the centre of this chamber by a sort of mesentery, which is perforated, so as to allow the free circulation of the blood which flows in this great sinus. The median division holds within it part of the œsophagus, the crop, the posterior salivary glands, and the aortic trunk. It is much and suddenly constricted in front, on its passage through the nervous collar to join the anterior, or buccal division, within which are found the anterior portion of the œsophagus, the buccal organ, and the anterior salivary glands.

The whole of this great sinus is lined throughout by a membrane, the peritoneal membrane of Milne Edwards, which is reflected upon all the organs it contains; and the mesentery, before alluded to, is formed by a duplicature of this same membrane. It is also seen distinctly forming a sheath to the aorta, which floats freely in the centre of the cavity, and it can be readily traced on all the organs, though it is for the most part incorporated with their tissues, so as to be scarcely, if at all, discernible.

Three branches from the venæ cavæ open into this great blood sinus; two into the posterior, and one into the median portion. The latter opens on the right side of the œsophagus, behind and close to the point where the aorta enters the chamber.

Now, the lining membrane of the sinus is continuous with that forming the wall of these trunk veins: and it is almost impossible to resist the conclusion that the great sinus results from the expansion and fusion of these venous trunks. Indeed, I should have been much inclined to adopt Delle Chiaje's conclusion, alluded to by Milne Edwards, that this is a veritable venous sinus, even had nothing else turned up to elucidate this interesting point: we have evidence, however, which appears sufficient to set this matter at rest.

Milne Edwards says that in the Calmars, or those Cephalopods with ten tentacles, the abdominal sinus has entirely disappeared, and that the lacunary portion of the circulation is in them confined to the