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LETTER XIII.

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.

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

Among the latter, he places the mollusca, 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 mollusca a sanies: but to call it anything 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.

Fig. 37.

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 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 carneæ), interlaced in every direction (Fig. 37).* 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, as the representative of the same in the vertebrate animals.

sac

The 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

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

+ Manuel de Malacologie, 131. "In the invertebrate animals, the heart and principal artery are generally placed on the upper part of the body, above the alimentary canal and largest portions of the nervous system; while in all vertebrate animals the order is reversed, the brain and spinal marrow being above, the heart below, the alimentary canal."-THOMSON in Cyclop. Anat. and Phy. i. 648.

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 arterial blood issuing from the heart (or hearts, where this organ is double) 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 branchiæ 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 mollusca.*

In the naked Cephalopoda there are three hearts. The true systematic heart, marked a in the diagram annexed (Fig. 38), 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

* "In the greatest number of mollusks the circulation is completed nearly in the same manner as in fishes, with this difference, however, that the heart is aortic instead of being pulmonary; or, in other words, the heart, receiving the blood from the breathing organs, sends it directly throughout the body. The heart is usually composed of a ventricle, whence the arteries take their origin, and of one or of two auricles in communication with the vessels which bring the blood from the gills; such is the case in the snails and all other Gasteropods, in the oysters and other bivalves; but sometimes there is no auricle, when we find a kind of venous hearts altogether distinct from an aortic ventricle, and situated at the base of the organs of respiration. Such is the case in the Octopus, the Sepia, and other Cephalopods. However this may be, in all mollusca the arterial blood passes through the heart, thence is distributed to every organ and part of the body, and then trends back to the organs of respiration, wherein having been subjected to the influence of the air, it returns anew to the heart to recommence its constant circuit."-MILNEEDWARDS' Elem. de Zool. i. 50.

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. Fig. 38.

a

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 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 (f) 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 artery (g), whose function is to carry it forwards to the gills or branchiæ (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. †

I 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 (x x, Figs. 38, 39) 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. 39), but of their use it is difficult to form an opinion. Cuvier makes several suppositions: they may be, he says, diverticula, 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

*Lister mistook them for ovaries. Exer. Anat. tert. xxxiv.

"The form of the systemic ventricle varies remarkably in the naked Cephalopods, as well as the direction in which it is extended; but there is great uniformity in the distribution of its vessels."—GRANT in Zool. Trans.