232 ANNELIDA. 



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 considerable 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 (6)*. 



(602.) 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 (fig. 112, g and e) run perpendicularly up- 

 wards, embracing the alimentary canal, and giving off branches at right 

 angles, which divide into innumerable ramifications, so as to cover the 

 whole intestine with a delicate vascular network ; these may be called 

 the deep-seated abdomino -dorsal branches. 



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

 from the termination of the dorsal vessel, with which it is evidently 

 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 that corresponds to it. From this system of superficial 

 vessels arises a cutaneous network, which traverses the skin in all di- 

 rections. 



(604.) Let us now trace the blood in its circulation through this 

 elaborate 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 anasto- 

 mosing 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-ganglionic trunks, therefore, the course of the blood 

 is necessarily from the head towards the tail ; and the circulating fluid 



* The moniliform character which these vessels exhibit is produced by the process 

 of dissection. If, in the ordinary way, a longitudinal dorsal incision is made, and 

 the two halves be then separated and pinned down, the vessels under such tension 

 are sure to assume a moniliform outline, that is, one part will dilate and the other 

 contract, and so on successively throughout the whole length of the vessels : the di- 

 lated portion will be filled with blood, and the contracted will be empty, and the 

 beaded figure will be perfect. If, however, a more careful mode of opening the 

 worm be adopted, dividing, by means of fine scissors, the membranous segmental 

 partitions, and laying gently open the integuments, these vessels will present a per- 

 fectly smooth outline : if, now, one of them be seized with the forceps and slightly 

 pulled, it will become irregularly knotted, or moniliform. Muscular fibres, chiefly 

 circular, are present in their parietes, and it is to the uneven action of these elements 

 that the beaded form is attributable. Dr. Williams. 



