212 THi: MTAI. I UNCTIONS. 



ration, numerous other vessels are provided in con- 

 nexion with the dorsal vessel, which, from its 

 greater power of contraction, approaches more i(f 

 the character of a heart. These vessels are, how- 

 ever, almost exclusively arterial ; and the blood 

 circulates in them only during a small part of its 

 course, being in other places effused into large cells 

 intervening between the extremities of the nutrient 

 arteries and the commencement of the short and 

 w ide brmicliio-cardiac veins, as they are called, pro- 

 vided for conveying the blood back into the dorsal 

 vessel. In the Crustacea, in like manner, there 

 occur large intermediate spaces in the systemic 

 part of the circulation ; but the vessels are of 

 greater size, and form more complete systems ; 

 inasmuch as they include a regular apparatus of 

 veins for collecting the blood from these cells into 

 central receptacles, or sinuses, as they are termed, 

 whence it is conveyed by a set of arteries to the 

 branchiae, and reconducted by a distinct set of 

 branchial veins to the heart; an organ which is 

 itself more developed than in any of the inferior 

 classes. 



In the Annelida, the vessels form together one 

 continuous system of canals; the large intercel- 

 lidar spaces of the preceding classes being here 

 replaced by a fine network of capillary vessels. 

 In the lower tribes of tliis class, the circulation is 

 carried on without the intervention of any distinct 

 organ of propulsion corresponding to a heart: but 

 in the higher annelida, several dilated portions of 

 the canal are met with, apparently performing the 

 function of that organ. The distinction between 

 the oxygenising and nutrient systems can here 



