204 CmCULATING SYSTEM. 



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 diffi- 

 culty in giving an intelligible view which would be applicable 

 to molluscous animals as a whole. We may, however, ob- 

 serve, that, in all, the arterial blood issuing from the heart 

 (or hearts, wdiere this organ is dou.ble) 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 throvigh 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 pecu- 

 liar kind, for a circulation through the Hver — the system, as 

 it has been called, of the vena portce, 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 projielled directly into a large artery or aorta (h), 

 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." — Milne- 

 Edwards' Elem de Zool. i. 50. 



