ANNELIDA. 205 



tant communication being established, through which the blood 

 passes freely from one to the other, by the intervention of seven or 

 eight pairs of large canals, situated in the immediate neighbourhood 

 of the generative apparatus, with which indeed they are interwoven. 

 Each of these voluminous vessels (d) is composed of a series of 

 swellings, or rounded bead-like vesicles, endowed with consider- 

 able contractile power ; and they form together a kind of heart of 

 remarkable construction, which propels the blood received from the 

 dorsal trunk into the ventral tube (b). 



Along the rest of the body, the communication between the 

 dorsal and ventral trunks is repeated at each ring by canals which 

 are much smaller than the bead-like or moniliform vessels, and 

 have no vesicular arrangement ; they (g and e) run perpendicu- 

 larly upwards, embracing the alimentary canal, and giving off 

 branches at right angles, which divide into innumerable ramifi- 

 cations so as to cover the whole intestine with a delicate vascular 

 net-work ; these may be called the deep-seated abdomino-dorsal 

 branches. 



The sub-ganglionic vessel (c) may be looked upon as arising 

 from the termination of the dorsal vessel, with which it is evi- 

 dently continuous at the anterior extremity of the body. At the 

 posterior edge of every segment, a delicate branch is given off from 

 this sub-ganglionic tube (/), which, running upwards in the same 

 manner as those derived from the ventral trunk, joins the dorsal, 

 and receives in its course a large anastomosing branch from the 

 deep abdomino-dorsal canal which corresponds to it. From this 

 system of superficial vessels arises a cutaneous net-work, analogous 

 to that described above as covering the digestive viscera which tra- 

 verses the skin in all directions. 



Let us now trace the blood in its circulation through this ela- 

 borate system. In the dorsal vessel (a) the sanguineous fluid 

 passes from the tail towards the head ; at the anterior extremity of 

 the body it passes partly into the sub-ganglionic vessel (c), through 

 the anastomosing branches, and partly into the ventral vessel (Z>), 

 into which it is forcibly driven by the contractions of the monili- 

 form canals. In both the ventral and sub-ganglionic trunks, there- 

 fore, the course of the blood is necessarily from the head towards 

 the tail ; and the circulating fluid is continually returned to the 

 dorsal canal by the deep and superficial abdomino-dorsal vessels 

 (e,/, g), completing the vascular circle. 



On reviewing the above arrangement, we immediately perceive 



