32 Natural History of Molluscous Animals : — 



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 distin- 

 guished 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 be- 

 tween 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 heait 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 nume- 

 rous ramifications, to conduct the blood through the branchiae 

 or gills, to be brought back by a corresponding set of vessels 

 to its point of departure. The circulation, therefore, is essen- 

 tially 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 vetia porta, — to which there is nothing 

 comparable in the MoUusca. 



In the Cephalopoda there are three hearts. The true 

 systematic heart, marked a in the diagram annexed {Jig. 25.), 

 consists of a single cavity, and is situated towards the centre 

 of the bod}-^, between the gills. By its action the blood is 

 propelled directly into a large artery or aorta (&), and into 

 two smaller vessels, to be distributed, by their joint ramifi- 

 cations, 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 ves- 

 sels converge and unite, by frequent anastomoses (inoscu- 

 lations), into larger ones, until they are collected into a few 



