In fact, the aorta having reached the spot where the digestive canal curves downwards to descend from the upper aspect of the pharyngeal bulb into the abdominal cavity, it plunges directly into a wide space or lacuna which surrounds the pharynx and occupies all the front part of the head, taking the place of the cephalic portion of the aorta; and the arterial blood poured by that vessel into this space directly bathes the brain, the muscles of the proboscis, and all the anterior part of the alimentary canal, after which it goes to supply the muscles of the foot and the cephalic appendages.

(1424). But there is one circumstance connected with this arrangement which appears even still more strange, namely that, while a portion of the general cavity of the body thus completes the vascular apparatus, the aorta to a certain extent acts as an abdominal cavity; for in its interior there is lodged a part of the digestive apparatus.

(1425). To ascertain this fact it is only necessary to slit open the aorta, the calibre of which is in this part as wide as a goose-quill; it is then seen that the large subcylindrical basis of the tongue, which projects from the posterior margin of the pharyngeal mass, is entirely enclosed within it. This organ, indeed, protrudes to a considerable distance into the interior of the arterial tube; and it is from the portion of the aorta which thus forms a sheath for the lingual apparatus that several arteries take their origin, the branches of which are distributed to the intestine and abdominal parietes, the orifices of which are discoverable when the tongue is withdrawn from its aortic sheath.

* "Observations sur la Circulation chez les Mollusques," par M, Milne-Edwards (Ann. des Sci. Nat. 1847).

Circulation of Haliotis (after Milne Edwards.)

Fig. 271. Circulation of Haliotis (after Milne-Edwards.) A, the head; B, the foot; C C, the two lobes of the mantle; D, mucus-secreting organ; E E,'the two branchiae; F, the anus. Beneath the rectum, that terminates at this outlet, is seen the orifice of the urinary apparatus; and a little further back, situated above the intestine, is the orifice of the generative apparatus. G, fold of intestine, which is lodged in a special compartment of the abdominal cavity, separated from that containing the stomach by a fibrous septum. H, the stomach, of which the anterior portion has been in a great measure removed. I, pharyngeal cavity laid open. J, abdomen.

* a, aortic ventricle surrounding the rectum.

* 6, the left auricle, into which opens the efferent vessel of the corresponding branchia, a portion of which is shown at E. The right auricle is seen immediately beneath the ventricle, and the corresponding branchia has been raised in order to show throughout its entire length the branchial vein or efferent canal, E, which runs along the adherent margin of the branchia, and brings arterialized blood from that organ to the heart.

* c, the great aorta, which arises from the posterior extremity of the ventricle and runs forward between the stomach and the intestine to discharge itself into the cephalic cavity.

* d, the abdominal artery, or posterior aorta, which arises from the commencement of the aorta and follows the convolutions of the intestine, to which, as well as to the liver, it furnishes branches.

* e, arterial sinus, into which the aorta empties itself. This is a great cephalic lacuna, limited above by the parietes of the pharynx, in front by the integuments and muscles of the head, and posteriorly by fibro-cellular bands. On injecting the animal by this cephalic chamber, the whole arterial system is immediately filled.

* f, the great artery of the foot, which arises from the cephalic sinus, and soon divides into four branches, which extend towards the hinder part of the foot. g, one of its lateral branches.

* h, afferent vessel of the left branchia. A little in front of the heart is seen the transverse canal, or common venous reservoir of the branchiae, which unites this vessel with its fellow, and which receives the veins from the rectum.

* i i, veins of the two lobes of the mantle in communication with a capillary network that extends along the base of the branchiae, and is proceeding to anastomose with the branchio-cardiac vessels.

* k, efferent vessels from the urinary gland opening into the common venous reservoir of the branchiae.

* I, venous canal of the shell-membrane or partition that extends from the walls of the abdomen to the margin of the shell.

* m, hepatic veins proceeding to open directly into the free space which surrounds the intestine, and which is continuous with the rest of the abdominal cavity. On the posterior part of the foot are seen veins which open into a system of lacunae situated upon the median line, and communicating with the abdominal cavity.

(1426). The inferior condition of the circulatory system in the Haliotls is, however, not indicated only by the singular arrangements described above. In that portion of the mantle which is adherent to the shell, and which forms a sort of border to the posterior and lateral parts of the body, arterial vessels seem to be altogether wanting, the whole circulation being apparently carried on by vessels which receive venous blood, derived immediately from the abdominal cavity, to which they partially return it, but at the same time convey a portion thereof into the branchio-cardiac vessels in the immediate vicinity of the heart. The fibrous tissue wherein these vessels are enclosed seems but little calculated to perform the functions of an accessory respiratory apparatus; so that it would appear, from this anatomical arrangement, that all the blood in progress towards the heart is not submitted to the influence of the air, and that it is a mixture of venous and arterial blood that is distributed by the heart throughout the arterial system.