102 F. W. GAMBLE, M.SC , AND J. H. ASHWORTH, B.SC. 



Vessels of the Body-wall. — This parietal system of true vessels is highly 

 developed in Arenicola marina. It consists of two longitudinal vessels 

 (1) the nephridial longitudinal vessel (Fig. 22, N. L. V.) running just 

 below the level of the nephridiopores, and (2) the more important dorsal 

 longitudinal vessel (Fig. 13, D.L.V.), which runs just above the level of 

 the insertion of the notopodial setal sacs. Both arise just behind the 

 first setae, and increase in size as they pass backwards. The former 

 receive vessels from the nephridia, just behind which they taper and 

 disappear. The latter, which may be traced to the anus, and are 

 largest in the branchial region, receive branches in each somite : (1) 

 from the segmental vessels (2) from its fellow of the opposite side. The 

 body- wall in the dorsal and lateral regions derives its blood-supply from 

 the nephridial and dorsal longitudinal vessels, and in the ventral region 

 from the neural vessels. These parietal vessels (Par. V.) run just within 

 the layer of circidar muscles in almost every groove between adjacent 

 longitudinal muscle-bands of the body- wall, are chiefly longitudinal in 

 direction, but at frequent intervals there are cross connections. 

 Branches from these vessels ramify between the bases of the epidermal 

 cells, and are accompanied by extensions of the coelom. 



Hearts. — The hearts are a pair of muscular bulbous swellings connect- 

 ing the visceral plexus with the ventral vessel on each side. Each 

 commences with the thin-walled expansion of the gastric vessel 

 ("auricle," Fig. 5, A. v.) which, after giving off the lateral oesophageal 

 branch, opens into the ventricle ( V.). The cavity of the ventricle is small 

 and broken up by a spongy mass of cells. The ventricular walls are 

 muscular, and contract from above downwards, forcing the blood 

 into the ventral vessel. (We have sometimes seen an apparent 

 reversal of the heart's action.) The spongy cardiac body arises by 

 ingrowths from the wall of the ventricle, chiefly in the middle and 

 ventral regions. It gradually encroaches on the blood spac?, so as to 

 reduce it considerably (PI. X., Fig. 36, Card. B.) in an old specimen. 

 The cardiac body in a young specimen (Fig. 38) is much smaller, and 

 extends obliquely across the heart, its general direction being down- 

 wards and backwards. The cells of the cardiac body in an old specimen 

 which we have examined are loosely arranged, so as to cause the forma- 

 tion of a large number of intercellular spaces, some of which are of 

 considerable size, and which are in life filled with blood (Figs. 36 — 38, 



