CIRCULATION IN THE ANNELIDA. 231 



pying the middle line of the lower, or abdominal 

 side of the body, and termed the abdominal vessel. 

 Often there are two such vessels ; one being super- 

 ficial, and another lying deeper ; the principal 

 nervous cord, and chain of ganglia being situated 

 between them. Frequently there are found, in 

 addition to these, vessels which run along the sides 

 of the body, and are therefore called the lateral 

 vessels. In every case there are, as we have seen in 

 the Planaria, numerous branches, and collateral 

 communications between the lateral, the abdominal, 

 and dorsal vessels ; more especially at the two 

 extremities of the body, where the great mass of 

 blood, which has been flowing in one direction in 

 one set of vessels, is transferred into others which 

 convey it in the contrary direction, and complete 

 the circuit of its course. The ramifications and 

 lateral connexions of the minuter branches are 

 often so numerous, as to compose a vascular net- 

 work covering a considerable extent of surface. 

 This general description of the circulatory system 

 is applicable to the tribes of Annelida possessing 

 the simplest structure, such as the Nais, the Am- 

 phinoma, the Nereis, and the Leech ; genera which 

 include a great variety of species of different shapes 

 and sizes. 



The blood moves forwards towards the head in 

 the dorsal vessels, and returns in the contrary 

 direction in the lateral and abdominal vessels, thus 

 completing its circuit. This is illustrated by the 

 diagram (Fig. 347); which exhibits the direction 

 of the stream in the Erpohdella vulgaris* an ani- 



* Hirudo vulgaris. (Linn.) Nephelis vulgaris. (Savigny.) 



