BARON CUVIER. 69 



of tlie differences of the organs of animals, he 

 observes, that the circulation of the blood fur- 

 nislies the most important variations. *' First, 

 there are animals which have no blood, such as 

 insects and zoophytes ; and, secondly, those 

 which have it, possess it in a double or simple 

 mode. That is called double circulation w^heu 

 no part of the venous blood can re-enter the 

 arterial trunk until it has made a certain circuit 

 in the organ of respiration, which must be 

 formed by the expansion of two vessels, the one 

 arterial and the other venous, nearly of equal 

 size, but shorter than the two principal vessels 

 of the body. Such is the circulation of man, 

 mammalia, birds, fishes, and many mollusca. 

 In simple circulation, a great part of the venous 

 blood re-enters the arteries without passing 

 through the lungs, because this organ receives 

 but one expansion from one branch of the ar- 

 terial trunk. Such is the circulation of reptiles. 

 There are yet other differences in the existence 

 and position of hearts, or muscular organs des- 

 tined to impel the blood. In simple circu- 

 lation there is never more than one ; but when 

 the circulation is double, one part is sometimes 

 seated at the base of the principal artery, and 

 the other at tliat of the pulmonary artery ; and 

 F 3 



