•CIRCULATION IN THE CRUSTACEA. 257 



superior classes of animals, performs so impor- 

 tant an office in the circulation of the blood, 

 namely, the heart: and this name, indeed, is 

 given by Cuvier, Morren, and others, to these 

 dilated portions of the vascular systems of the 

 higher orders of Annelida. 



Here, also, the statements of different anato- 

 mists are at variance, with regard to the direc- 

 tion taken by the blood while circulating in the 

 vessels ; Home and Duges represent it as pro- 

 ceeding forwards in the dorsal, and backwards 

 in the abdominal vessels; a course which im- 

 plies its descent along the lateral communicating 

 vessels just described ; while De Blainville and 

 Morren ascribe to it a course precisely the 

 reverse. Amidst these conflicting testimonies, 

 it is extremely difficult to determine on which 

 side the truth lies ; and a suspicion will natu- 

 rally arise, that the course of the blood in the 

 vessels may not be at all times uniform, but may 

 be liable to partial oscillations, or be even com- 

 pletely reversed, by the operation of particular 

 disturbing causes. 



The larger Crustacea possess a circulatory 

 apparatus still more extensive and complete, 

 accompanied by a corresponding increase in the 

 energy of the vital functions. As we follow this 

 system in the more highly organized tribes of 

 this class, we find the powers of the dorsal 

 vessel becoming more and more concentrated 



VOL. II. s 



