PLANS OF CIRCULATION. 213 



scarcely be drawn ; for the former consists only 

 of occasional branches proceeding from the latter 

 system, and again joining it, so as to constitute 

 short collateral circles. 



In the Mollusca, and all the higher classes, the 

 circulating system is found to attain still greater 

 perfection and a more ample developement ; the 

 blood is always conducted through continuous ves- 

 sels, and urged onwards by a force derived from the 

 contractions of a heart. The position of this organ, 

 however, differs greatly in different cases. In the 

 generality of Mollusca there is only one heart, and 

 it is always aortic or sijstemic ; that is, placed at 

 the origin of the nutrient system of arteries, (a. 

 Fig. 3.]2). In some instances, there are two aortic 

 hearts, placed symmetrically on the two sides of 

 the animal ; and in one order, besides the single 

 aortic heart, there are also two branchial hearts ; 

 that is, hearts at the origin of the branchial or 

 respiratory arteries, {b.) In Fishes^ again, this 

 arrangement is reversed ; the heart, which is always 

 single, being branchial, not aortic. In Reptiles, 

 the two systems are in some measure intermixed 

 by means of cross branches of communication at 

 different points :* the heart being placed near the 

 origin of these branches, or being itself the medium 

 of this communication. In all warm-blooded Ver- 

 tebrata, that is, in Birds and Mammalia, there are 

 provided two hearts, one belonging to each system, 

 and forming by their junction a double heart. 



Having premised this general sketch, T proceed 

 to describe each variety more in detail. 



* There is here an analogy between Reptiles and Annelida ; for 

 in both, the respiratory circ-ulation is only a branch of the systemic. 



