ANNELIDA. 



placed along the back, and these external vascular tufts communi- 

 cate with delicate plexuses of vessels situated in the interior of the 

 body, called the branchial plexuses. In the figure the branchial 

 plexuses of the left side only are represented (</, </, <?), and of 

 these one marked q' has been turned aside. The blood and nutri- 

 tious fluids derived from the whole alimentary tract are collected 

 by the large ventral intestinal vein (w, w, n), and conveyed to the 

 branchial plexuses through the numerous vessels (o, o, o), some of 

 which (o', o', o') are displaced in the drawing in order that their 

 connections may be better seen. Besides the blood and nutriment 

 thus derived from the intestine, the branchial plexuses receive the 

 circulating fluid from all the segments of the muscular envelope by 

 separate veins (p 9 p), and thus the blood from all parts is brought 

 to the gills and exposed to the influence of oxygen. 



After undergoing respiration, the blood is collected from the 

 branchial plexuses by the lateral veins (r, r, r) ; from which, 

 through communicating vessels (s, s, s), it passes into the aorta 

 or great dorsal vessel (, , ), to be distributed through the body. 

 From the aorta large trunks (v, v) are given off to form the intes- 

 tinal artery (w, w), which, ramifying over the intestine, communi- 

 cates with the intestinal vein (w, w), and thus completes the vas- 

 cular circle.* 



In the JVereidce, the aorta, or dorsal vessel, runs along the whole 

 length of the back, and in each ring offers a perceptible fusiform 

 dilatation, so that it has a beaded appearance ; at every segment it 

 gives off lateral branches, every one of which is furnished with a 

 little rounded vesicle, which Delle Chiaje conceives to be a distinct 

 heart or contractile cavity, calculated to assist in the propulsion of 

 the contained blood. 



In Arenicola the arrangement of the vascular trunks seems to 

 be very nearly similar to that found in the earthworm ; but, instead 

 of the moniliform hearts, ( 247,) two large contractile sinuses 

 communicate between the dorsal and ventral vessels {Jig. 94, 6, b). 



(263.) The reproductive organs of iheDorsibranchiateAnnelidans 

 are, perhaps, less known than those of any other animals. Cuvier-)- 



* The parts indicated in the drawing by letters not referred to in the text are the 

 following : a, a, the ventral surface of the segments of the body ; e, e, the ventral 

 oars or packets of bristles ; /, /, the ventral cirri, or feelers ; g, the anal cirri ; ft, the 

 anus ; i, i, k, k, the bases of the dorsal and ventral oars, with their surrounding mus- 

 cles ; J, /, the dorsal longitudinal muscular bands ; m, m, the ventral longitudinal 

 muscular bands. 



t Le9ons d'Anatomie Comparee, vol. v. p. 186. 



