ANNELIDA TERRICOLA. 



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

 Each of these voluminous vessels is composed of a series of 

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

 siderable 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. 



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 ves- 

 sels, and have no vesicular arrangement ; they run perpendicu- 

 larly upwards, embracing the alimentary canal, and giving off 

 branches at right angles, which divide into innumerable rami- 

 fications so as to cover the whole intestine with a delicate vas- 

 cular network. These may be called the deep-seated abdomino- 

 dorsal branches. 



The sub-ganglionic vessel 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 

 oft' 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 cuta- 

 neous network, analogous to that described above as covering 

 the digestive viscera, which traverses the skin in all directions. 



Now let the blood in its circulation be traced through this 

 elaborate system. In the dorsal vessel the sanguineous fluid 

 passes from the tail towards the head ; at the anterior extrem- 

 ity of the body it passes partly into the sub-ganglionic vessel, 

 through the anastomosing branches, and partly into the ventral 

 vessel, into which it is forcibly driven by the contractions of 

 the moniliform canals. In both the ventral and sub-gangli- 

 onic trunks, therefore, 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 super- 

 ficial abdomino-dorsal vessels completing the vascular circle. 



On reviewing the above arrangement, it is immediately per- 

 ceived that, notwithstanding the similarity observable in the 

 distribution of the ventral and sub-ganglionic systems of ves- 

 sels, in a physiological point of view, they are subservient to 

 very different functions ; the former representing the systemic, 



6 



