arteries and veins, is met with in many of the 

 Testacea, as the snail ; and, among the Mollusca, 

 the cuttle appears to have no fewer than three 

 distinct hearts. In the former, the heart is situ- 

 ated immediately behind the respiratory organ, 

 from which it directly receives its blood, and 

 propels it by the main artery of the body to every 

 part, whence it returns by the chief veins to the 

 respiratory organ, and thence again to the heart. 

 In the latter, while one of the three hearts, like that 

 of the snail, receives the blood from the respira- 

 tory organs, and transmits it in a similar manner 

 by the arteries and veins, the other two are situ- 

 ated at the extremities of the two chief veins, and 

 serve to propel it through the respiratory organs 

 preparatory to its again arriving at the first heart. 

 In insects in general, the progress towards per- 

 fection of the vascular system appears to be re- 

 trograde, but it is so in furtherance of an essen- 

 tial purpose in their economy that of rendering 

 them light, and therefore capable of flying. Ac- 

 cordingly, in the majority of these animals, the 

 nutritive matters appear to be taken up by a kind 

 of imbibition from their intestinal canal, and to 

 become accumulated in one large blood-vessel, 

 which runs along the back from head to tail, and 

 from which no other vessels, except in some few 

 insects, as the spiders, can be observed to arise, 

 for the purpose of carrying the blood to the seve- 

 ral parts of the body. We must suppose, there- 

 fore, that, as it is received by imbibition, so it is 

 transmitted for the purposes of nourishment, by 



