234 THE VITAL FUNCTIONS 



branchiae ; the other, consisting of the branchial 

 apparatus itself, which intervenes between the dorsal 

 and the abdominal vessels, and which, by its move- 

 ments mechanically promotes the circulation in 

 the latter of these systems. An arrangement the 

 reverse of this is met with in the Arenicola, where 

 the same mechanism by which the respiratory 

 organs are set in motion, supplies the force that 

 impels the blood in the dorsal vessel, and the system 

 of vessels derived from it : while the current of the 

 blood in the abdominal system is quickened by the 

 pulsation of two contractile reservoirs, which per- 

 form the functions of hearts. 



The tribe of Lumbrici, which includes the earth- 

 worm, is distinguished from the annelida already 

 noticed, by being more highly organized, and pos- 

 sessing a more extensive circulation, and a more 

 complicated apparatus for the performance of this 

 function. The greater extent of vascular ramifica- 

 tions appears to require increased powers for carry- 

 ing the blood through the numerous and intricate 

 passages it has to traverse ; and these are obtained 

 by means of muscular receptacles, capable, by their 

 successive contraction, of adding to the impulsive 

 force with which the blood is driven into the trunks 

 that distribute it so extensively. These muscular 

 appendages are globular or oval dilatations of some 

 of the large vascular trunks, which bend round the 

 sides of the anterior part of the body, and establish 

 a free communication between the dorsal and the 

 abdominal vessels. They are described by Duges 

 as consisting, in the Lumbriciis gigas, of seven 

 vessels on each side; forming a series of rounded 



