Fig. 270. Vena cava of Aplysia laid open.

(1420). In Aplysia, the arterial blood, having been distributed throughout the body by means of the heart and aortic vessels, is received into a capillary system, which forms a rich network composed of minute vessels, the walls of which are perfectly distinct; but these capillaries are found not to be continuous with any system of recurrent vessels, but gradually resolve themselves into little lacunae formed amongst the interstices which occur between the bands of cellular membrane and the fibres of various tissues. These vacuoles communicate in their turn with larger lacunae, situated beneath the common integuments, or occupying the interspaces between the muscular fasciculi of the foot of the mantle, and of other parts of the body. The result of this arrangement is the formation of a vast system of venous cavities, dispersed throughout the abdominal parietcs. In the foot and in the lobes of the mantle these lacunae are very dilatable, and afford space for a considerable accumulation of fluid; in the dorsal region, on the contrary, they are small, and more densely congregated. It is this structure which constitutes the aquiferous system of Delle Chiaje; but it has no communication with the exterior of the body.

The membrane, which imperfectly lines the abdominal cavity, separates this structure from the visceral chamber, but does not cut off the communication that exists between them; on the contrary, the peritoneal tunic is itself of a spongy texture, and is perforated with numerous apertures, whereby a free passage is established between the subcutaneous lacunae and the interior of the abdomen. In this way it happens that, when a coloured fluid is injected into the visceral cavity, the whole lacunary system becomes filled; and on throwing injections, even of coarse materials, into the muscular interstices of the foot or mantle, they are seen at once to diffuse themselves through the abdominal cavity.

(1421). From the above and similar facts, Milne-Edwards has satisfactorily established the following important conclusions: -

1st. That no complete vascular system exists in any of the Mollusca.

2nd. That throughout a greater or less extent of the circulatory circle veins are entirely wanting, their functions being performed through the medium of lacunae, or by the great cavities of the body.

3rd. That frequently the veins are wanting altogether, and that in such cases the blood distributed through the body by the arterial system can only return to the respiratory surface by the intervention of the interstitial lacuna) above described.

(1422). Professor Huxley, in a letter addressed to Professor Milne-Edwards*, relative to the circulation of the blood, expresses himself very decidedly upon this important point in the anatomy of the Mollusca. In Firola, one of the Heteropod division, he observes that, owing to the perfect transparency of the body of this mollusk whilst alive, nothing is more easy than to observe the circulation of the blood throughout its entire course. In this creature no veins whatever are observable. The globules of the blood may be seen to issue in crowds from the open termination of the arteries of the foot, through the substance of which they immediately become diffused, and may likewise be observed to pass from the mass of the mouth, in which the aorta terminates, directly into the peri-intestinal cavity, in which they may be seen to return gently, frequently stopping in their course towards the heart. Occasionally some of them may be traced directly into the auricle, passing through the interspaces between the network of muscular fibres composing its walls1 . in the meshes of which they maysometimes be observed to stop for a short period.

When the animal begins to grow weak, and the circulation becomes enfeebled, it is even possible to follow with the eye any given globule during its passage through the peri-intestinal cavity, and through the heart into the aorta.

* Ann. des Sci. Nat. 1850.

1 In Firola, Professor Huxley assures us, the walls of the auricle of the heart are composed of a kind of lacework made up of striated and ramified muscular fibres, between which large open spaces arc observable.

(1423). In studying the anatomy of Haliotis, Milne-Edwards* observed that, although injections thrown into the heart were easily made to fill the general arterial system, so as to exhibit the arteries supplied to the liver, to the stomach, and internal viscera generally, and also to, render visible even the capillary vessels, in the head he invariably found the injection extravasated so as to fill a great cavity, in which were lodged the brain, the salivary glands, the pharynx, and all the muscles belonging to the oral apparatus. At first it was supposed that this extensive extravasation was caused by some rupture of the vascular parietes; but after many unsuccessful attempts it was at last discovered that, on attempting to follow the course of the aorta into the head, it was impossible to find any trace of that vessel beyond the point where this extravasation invariably began to show itself: at this place, indeed, the walls of the great artery entirely disappeared, or, rather, became confounded with the membranous septum that here separates the abdomen from the cephalic cavity: neither could any continuity be traced between the arterial trunk, after its entrance into this extensive cavity, and the arteries proceeding from it to ramify in the fleshy portion of the foot, although these latter were invariably well filled with the coloured injection employed; and it soon became evident, from numerous observations, that in this Gasteropod a free communication is normally established between the great arterial trunk of the body and the cephalic cavity, wherein are lodged the principal nervous centres and the whole anterior portion of the digestive apparatus, and that this cavity, in the living animal, is filled with arterial blood.