ART. VI. An Introduction to the Natural History of Molluscous Animals. In a Series of Letters. By G. J.

Sir,

Letter 9. On their Circulating System.

ARISTOTLE divides animals into those which have blood, and those which have none; and these primary classes were appropriately named the sanguineous and exsanguineous. Among the latter he places the Mollúsca, as all naturalists did for a long time afterwards, and as all, except naturalists, continue to do. Blood is scarcely known to the vulgar unless by its red colour; and so essential is this character deemed, that it appears to them little less than an abuse of language to apply the term to any white or colourless fluid. Even Linnæus seems to have participated of this prejudice, and to have yielded to its influence, when he called the circulating fluid of the Mollúsca a sanies: but to call it any thing else than blood is apt to lead into error; for it possesses all the essential properties of blood, flows in an analogous circle of vessels, and answers the same purposes in the system.

The circulating system of the Mollusca consists of a heart, either single, or with its parts disjoined; and of two kinds of vessels, viz. arteries and veins: and the latter are supposed to perform the additional function of absorbents; for nothing

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analogous to these has been yet detected. The heart is very various in point of figure, but is always evidently muscular, and has its interior strengthened with fleshy cords (columnæ carneae), interlaced in every direction. (fig. 24. *) It is placed in general in the back, above the alimentary canal, near to or between the branchiæ, and in a cavity usually called the pericardium, and considered, according to Blainville erroneously (Manuel de Malacologie, &c., p. 131.), as the

representative of the same sac in the vertebrate animals. The

* Interior view of the heart of Octopus vulgàris, from Cuvier. a, The aorta; b, branchial veins; c, the valves; d, columnæ carneæ.

arteries are very elastic, and probably muscular, although no fibres can be detected in their gelatinous structure; their coats are thicker and stronger than those of the veins, which, indeed, are so extremely thin as frequently scarce to be distinguished from the tissues in which they run. The veins do not appear to be provided with valves, as you know the veins of other animals are; but valves are placed at the orifices between the cavities of the heart, and very often at the entrances into the primary arterial and venous trunks.

With regard to the distribution of the sanguiferous vessels, it will be necessary to give a sketch of it in the principal orders separately; for it is subject to such important and considerable modifications, that there would be great difficulty in giving an intelligible view which would be applicable to molluscous animals as a whole. We may, however, observe, that, in all, the blood issuing from the heart is distributed through the body by the medium of the arteries, and returned towards the centre by the veins, which have united there into one or a few trunks; whence, again, they diverge into numerous ramifications, to conduct the blood through the branchia or gills, to be brought back by a corresponding set of vessels to its point of departure. The circulation, therefore, is essentially the same as in the vertebrate animals; but there exists in the latter an arrangement of vessels of a very peculiar kind, for a circulation through the liver-the system, as it has been called, of the vena porta, to which there is nothing comparable in the Mollúsca.

In the Cephalopoda there are three hearts. The true systematic heart, marked a in the diagram annexed (fig. 25.), consists of a single cavity, and is situated towards the centre of the body, between the gills. By its action the blood is propelled directly into a large artery or aorta (b), and into two smaller vessels, to be distributed, by their joint ramifications, to every organ and point of the body. One of the small arteries comes off from the inferior surface, and is destined to supply the testicle or ovary; the other rises from the anterior surface, and supplies in part the gills, the sac, and more especially the intestines and chylopoietic viscera; but it is the aorta, issuing from the heart on the posterior side, which carries the great mass of blood through the system, to furnish new materials for its growth and secretions. From the extreme branchlets of the arteries the blood flows on into the capillary extremities of the veins, and commences its return to the centre; for the small branches of the latter vessels converge and unite, by frequent anastomoses (inosculations), into larger ones, until they are collected into a few

trunks. The veins of the feet and superior parts form ultimately two of these (c), which almost immediately coalesce

into one greater (d); and this vessel, after descending through part of the viscera into the abdomen, and receiving blood from various little tributaries, again divides into two branches (e e). Each branch is here joined by a vein (o) of a size equal to itself, and which has brought the blood from the abdominal viscera; and a little afterwards by another, from the cloak and the supports of the gills. When thus augmented, they proceed to their termination in the lateral hearts, placed, one on each side, at the root of the branchiæ. These hearts (ƒ) are called pulmonic; they are rather cellular than fleshy in texture, moderately thick, of a blackish grey colour in some genera, pale red in others, and pitted internally with many little cavities communicating together. Two large valves are placed at the venous orifice, to prevent regurgitation; but there is none at the orifice by which the blood enters the VOL. V. No. 23.

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artery (g), whose function is to carry it forwards to the gills or branchia (h), where, circulating through the windings of their beautiful leaflets, it is purified, and thence returned by veins running in the reverse direction, and which open at last by a single trunk (i) into the systemic heart, again to run the same endless circuit.

have omitted in this description a very remarkable peculiarity connected with the venous system, and which merits our particular notice. Previously to their junction with the pulmonic hearts, the two branches into which the great dorsal vein bifurcates, and their accessory veins, pass across two large cavities, called venous by Cuvier, which communicate externally by an aperture on each side near the gills. In this part of their course the veins are garnished with some very singular glandular bodies (xx, figs. 25. and 26.), of a spongy cellular structure and yellow colour, from which an opaque yellowish mucous secretion can be easily pressed in considerable quantity. The cells of these bodies open freely into one another, and they have likewise a very free and direct communication with the interior of the veins to which they are appended (fig. 26.); but of their use it is difficult to form an

opinion. Cuvier makes several suppositions: they may be, he says, diverticula [side-paths], in which the venous blood is more fully exposed to the purifying influence of the circumfluent water; or they may be excretory canals, by which the spongy glands pour into the

vein some substance which it could not of itself extract from that fluid; or, on the contrary, they may be emunctories, by means of which the blood is purged of some noxious principle. This last conjecture, he thinks, is rendered more probable by the abundance of the yellowish mucus poured out; and it is certain that the communication between the interior of these bodies and the medium in which the animals live is very open; for when air or an injection is thrown into the vein, the air or the injection passes very readily through the glands into the venous cavity, and thence outwards; or, on the contrary, if air is blown by its external orifice into this cavity, it

"Were it practic

passes thence very often into the veins. able," says Dr. Fleming, "to analyse the yellow mucus which these glands contain, some light might be thrown on the subject: indeed, it appears not improbable that this arrangement is analogous in its functions to the urinary system in the most perfect classes." (Philosophy of Zoology, vol. ii. p. 426.)

The circulatory apparatus of the Gasterópoda is less complex than that of the preceding order. They have a single heart, the position of which in the body is regulated by the position and symmetry of the branchiæ; for, in molluscous as in vertebrate animals, the heart is never far distant from the aërating organs. In the greater number of the Gasterópoda it is situated in the back, above the intestinal canal, at an equal distance from each gill when this is paired, or obliquely to the left, and rarely to the right, when it is single. It is composed of an auricle and a ventricle: the former cavity is very variable in shape, and has very thin but muscular walls; the latter is equally variable, but, in general, of greater capacity, and more decidedly muscular. It is from one of the extremities of its great diameter that the arterial or centrifugal system proceeds; sometimes by a single trunk, or more commonly by two vessels. Of these, one is anterior, and the other posterior: the first furnishes branches to the head, to the gullet and adjacent organs; while the second sends its ramifications to the stomach, intestines, the liver, and the secretory organs of generation. The blood is brought back from these distant parts, as in other animals, by the venous, or, as it has been happily designated, the centripetal system; the numerous branchlets of which, after repeated inosculations, are at length united into one large trunk, which, generally without the intervention of any dilatation or auricle, assumes the character and office of a pulmonary artery, that again divides and subdivides itself, to conduct the circulating flood through all the sinuosities of the gills.

The description just given is liable to many exceptions, were we to descend to particular families and genera; and, although in a sketch of the kind I attempt to give you, it is impossible to notice all their peculiarities, yet it may be useful, and not void of interest, to select a few examples illustrative of the most remarkable anomalies in the arrangement of their circulating system. The Tèthys leporina (fig. 27.*), a native of the shores of the Balearic Isles, will afford our first instance.

* From Cuvier's Mémoires pour servir à l'Histoire et à l'Anatomie des Mollusques; the most valuable work by far in this department of natural history.