CIRCULATION IN THE ANNELIDA. 251 



is also a corresponding trunk, occupying the 

 middle line of the lower, or abdominal side of 

 the body, and termed the abdominal vessel. This 

 latter vessel is sometimes double ; one being su- 

 perficial, 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 la- 

 teral, 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 con- 

 nexions 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 possess- 

 ing the simplest structure, such as the Nais, 

 the Nereis, and the Leech; genera which in- 

 clude a great variety of species of different 

 shapes and sizes. 



Although the vessels themselves may be 

 plainly discerned, it is not so easy to determine 



