TEKEBELLA NEBULOSA. 



271 



Fig. 135. 



which likewise communicates with the dorso-intestinal vessel by nume- 

 rous anastomosing ramifications. 



(692.) The vessels above enumerated may be considered as consti- 

 tuting the general venous system of the body ; and the blood which 

 they convey to the dorsal trunk is, 

 by the contractions of that vessel, 

 in great part distributed to the 

 branchiae through three pairs of 

 branchial arteries derived imme- 

 diately from the dorsal heart. Still, 

 however, all the blood thus moving 

 from behind forwards is not con- 

 veyed to the branchial organs, since 

 a certain portion finds its way 

 through a small median vessel to 

 the labial organs and cephalic ten- 

 tacula. 



(693.) After having passed 

 through the branchial organs, the 

 renovated blood is received by ves- 

 sels which unite to form a median 

 trunk (fig. 135, o) that runs be- 

 neath the alimentary tube and im- 

 mediately above the ventral chain 

 of nervous ganglia. This ventral 

 trunk is continued along the whole 

 length of the body, and gives oft 7 

 opposite to each ring a pair of trans- 

 verse vessels, which, after having 

 supplied branches to the integu- 

 ment and locomotive organs, bend 

 upwards, to be distributed over the 

 walls of the intestine, where their 

 ramifications contribute to form the 

 vascular network above alluded to. 



(694.) The ventral vessel and its ramifications fulfil, therefore, the 

 functions of an arterial system ; and consequently the branchiae them- 

 selves must be regarded as the agents employed in propelling the blood 

 through the systemic circulation. These organs, indeed, may be ob- 

 served, at intervals, to contract with considerable energy, and thus 

 materially to assist in urging the blood through the arterial rami- 

 fications. 



(695.) M. de Quatrefages observes* that both in the Erratic andTubi- 

 colous Annelidans the sexes are separate, and states that the generative 

 * " Memoire sur les Hermelliens," Ann. des Sci. Nat. 3 ser. 1848. 



Arrangement of the vascular system in 

 Terebella. (After Milne-Edwards.) 



