In this singular and beautiful creature, the heart is situated in the middle of the back, immediately under the skin. Its

a, Veil; b, tentacula; c, neck; d, organs of generation; e, anus and another excretory orifice; f, greater branchiæ; g, lesser branchiæ; h, margins of foot.

oval and very thin auricle receives the branchial veins, which trend towards it like the spokes of a wheel towards the nave, and pour into it the purified blood, not by one or two, but by numerous orifices. The opening from the auricle to the ventricle is furnished with two very distinct valves; and, as usual, the latter cavity is more fleshy and opaque than the auricle, of an oval form, and strengthened with small muscular cords. Two principal arteries take their departure from it; one of which, proceeding forwards, gives branches to the stomach, œsophagus, to the organs of generation, to both sides of the back and foot, and lastly loses itself in the veil; while the other artery, directing its course backwards, is principally distributed on the rectum and liver. The veins issuing from this organ, and from the intestines, run in the sides of the body, where, in conjunction with the veins returning from the

foot, the back, and the veil, they form ultimately two main vessels to carry the blood to the branchiæ; which are external, and arranged like crests, in two rows along the back, the principal ornaments of the animal.

We may take our next example from among the terrestrial Mollúsca. The heart of the slug (Lìmax àter) is placed almost on the middle of the pulmonary cavity, "included in an extremely thin bag or pericardium, in whose cavity there is abundance of watery moisture, as clear as the purest crystal." The auricle is of a triangular figure; the apex resting on the superior surface of the oval ventricle, and the much dilated base receiving the pulmonary veins, which, like those of the Tethys, open into it by many mouths. But the peculiarity most worthy of notice in this animal is the colour of its arteries; an opaque and pure white, like what it would be were we to suppose them filled with milk, and rendered very obvious by the darkness of the grounds upon which the vessels trace their course; as, for example, in the intestines, which are of a dark green; or in the liver, which is of a blackish brown colour. The finest injections do not produce any thing, adds Cuvier, more agreeable to the eye of the anatomist than the white ramifications of the arteries in the black slug.

'The most singular deviations from the normal structure and disposition of the blood-vessels in the Gasterópoda are, however, to be found in the celebrated Aplýsia. In this mollusque, the great branchial vein receives the aërated blood from its little tributaries, which penetrate it in such a manner that their orifices form imperfect circles on the inner surface. The vein itself runs along the convex border of a crescent-shaped membrane, supporting the branchiæ, and opens, as usual, into the auricle; remarkable for size and the thinness of its parietes [walls], which resemble fine gauze, the very slender fleshy filaments forming a pretty network. The ventricle is oval, and its walls are also thin, although furnished with fleshy columns, crossed in every direction. The aperture between it and the auricle is provided with two valves, which hinder any reflux of blood. The aorta proceeding from the ventricle divides into two trunks; the first, trending directly to the left, pierces the pericardium, after a very short course, to enter the abdomen; the second returns at first towards the right, sends off a branch, and then leaves the pericardium also at its right side. The portion enclosed in this cavity has attached to it two crests composed of small vessels, which rise from the trunk itself, and again reenter it, without affording the anatomist any clue whereby to guess the use of such a curious

formation. It is always easy to inflate or inject these crests; and Cuvier hazards a conjecture that they may be secretory organs for the production of the liquid which fills the pericardium.

But a still more extraordinary peculiarity remains for our notice. The large vessel which carries forward the venous blood to the branchiæ, and which may be named either a vena cava, or a branchial artery, since it fulfils the functions of both, after sending off arterial branches to the leaflets of the gills, remains for a certain space smooth and entire; but one part curves itself to the left, and another to the right, and these two branches assume suddenly a new form and structure; they become, in fact, absolutely confounded with the great general cavity of the body. Their walls are now formed of transverse and oblique muscular ribands, which cross in every direction, but leave between them apertures visible to the naked eye, and permeable to all sorts of injection; thus establishing a free communication between these vessels and the abdominal cavity, so that the fluids contained in the one can readily permeate into the other. This structure is so anomalous, that Cuvier was for some time doubtful of the accuracy of the dissections which seemed to prove it; but at last he fully satisfied himself, and ascertained distinctly that there was no other vessel to carry the blood to the branchia except the muscular and perforated cavities just described, and into which all the veins of the body open directly or indirectly. It follows, therefore, that the fluids shed into the abdomen can mix directly with the mass of blood, and be carried to the branchia with it; and that the veins perform the office of absorbent vessels. This vast communication, says the great naturalist from whom I borrow these anatomical details, is, doubtless, the first step to that still greater which nature has established in insects, where there is no particular vessels for the nutritive fluid; and we have already seen a trace of it in the Cephalopoda, where the venæ cavæ and the abdominal cavity communicate together through the medium of certain spongy glands.

In the genus Orchídium, a naked mollusque, the venæ cavæ exhibit a formation in some respects similar to that in Aplýsia; but I pass over this, to notice another sort of deviation from the common, in the ear shell (Haliòtis), and in some simple univalves; as, for example, in Fissurélla. The heart in those genera is provided with two auricles; one of which receives the vein carrying the purified blood from the right branchiæ, and the other that from the left. The auricles open into the ventricle each by a single and generally narrow orifice; but

in the Chitónidæ each auricle has two distinct and separated ventricular orifices; of which, according to Cuvier, there is no similar example to be found in the animal kingdom. Further, in these genera, the ventricle, or proper heart, is perforated by the straight gut, or, in other words, the heart encircles that intestine; a peculiarity not to be observed in other Gasteropoda.

What, however, is the exception and anomaly among Gasterópoda, becomes the usual formation in the Conchífera or bivalved Mollúsca; for in by much the greater number of them the gut passes through the heart, or rather, as Blainville explains it, the heart is curved round, the rectum, in such a manner that the two extremities of its transverse diameter seem to touch. All the bivalved Mollúsca have also two auricles; one placed on each side of the ventricle, and opening by a narrow neck into it. They receive the blood through a branchial vein coming from each gill, and transmit it to the ventricle, which is an oval or spindle-shaped bag, situated in the medio-dorsal line.* From the ventricle two aorta depart: a posterior and lesser one, which passes under the rectum, and distributes its branches to the posterior parts of the body; and an anterior one, which runs forward even to the anterior adductor muscle, furnishes branches to the stomach, to the liver, the foot, and adjoining parts, then curves downwards by an anastomosing branch, which follows the margin of the cloak, to meet and unite with a similar branch from the posterior aorta; forming thus a great arch, of which the inferior branches go to the tentacular fringe of the cloak, while the others, of greater size, remount and ramify over the whole surface of that organ.

Of the centripetal system we find that the venous radicles of the belly and of the anterior parts of the body unite into two large trunks, which issue from the hepatic region underneath the rectum, and, after having received two veins which have followed the margin of each lobe of the cloak, they open into the anterior end of a sort of auricle or venous reservoir, placed longitudinally below the heart in the dorsal line. This reservoir receives at its posterior end two other rather large veins, which have gathered the blood and brought it back from the posterior parts of the body, and even from the margins of the cloak. From this reservoir the branchial arteries likewise originate: they are two in number, one on

In the oyster," the auricle and ventricle are very thin in their coats; so much so as to make them unequal to apply force to the blood; but the ventricle is laterally connected to the great muscle, whose action will accelerate the circulation." (Home's Comp. Anatomy, vol. iii. p. 160.)

each side, of considerable size, and direct their course along the superior margins of the branchial lamina, to which they furnish numerous branches. These form two series; one for the internal face of the external leaf, and the other one for the external face of the internal leaf of the branchiæ; but in consequence of the many vertical and anastomotic branchlets, a close vascular network is the result of the whole arrangement. The veins, drawing their supply from this network, run backwards in a direction parallel to the arteries, and form a similar network, but on the opposite faces of the branchiæ.

As it offers some exceptions to this account of the distribution of the blood-vessels in the Conchífera in general, I am tempted to extract f for for you the interesting description of the circulation in Terèdo navalis, as given by Sir Everard Home : "The heart," he says, "is situated upon the back of the animal, near the head; consisting of two auricles (fig. 28.*), of a

* Figure of Terèdo navàlis, showing the heart and other internal organs, of the natural size, exposed in a posterior view. aa, The boringshells, separated and turned back; b, the digastric muscle; c, the intestine passing over it; dd, the testicles; e e, the auricles of the heart; ff, the ventricle; g g, the artery going to the head; hh, the vessels from the branchiæ going to the heart; ii, the branchiæ or gills; kk, ducts of the testicles, traced through their course; 11, a strong substance, with transverse fibres, having a pile upon it, to strengthen this, the weakest part of the animal.