18 F. W. GAMBLE AND J. H. ASHWOBTH. 



the vessel running in the second diaphragm, (3) from a vessel 

 just behind the third diaphragm, (4 and 5) from the vessels to 

 the second and third nephridia. Near the middle of each 

 somite the two neural vessels are united by cross connections, 

 which also supply the nerve-cord (Pis. 2, 3, fig. 13, N. V., 

 N. C. v.). 



Behind the third diaphragm the neural vessels supply the 

 oblique muscles by branches which run the whole length of 

 the bands, and are connected with the outer longitudinal 

 parietal vessel (fig. 13). 



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

 vessels is highly developed in Arenicola marina. It con- 

 sists 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 back- 

 wards. 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 circular 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 swell- 

 ings connecting 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 

 o-iving off the lateral oesophageal branch, opens into the ventricle 



