16 P. W. GAMBLE AND J. H. ASHWORTH. 



which are attached to the anterior face of the caudal septa, and 

 which run downwards and forwards to open into the ventral 

 vessel (PI. 2, fig. 5). Of these there may be twenty-seven to 

 thirty pairs. In front of the caudal region each of the last 

 seven pairs of gills returns an efferent branch to the dorsal 

 vessel, and between these there are three or two pairs of 

 smaller branches which run round the alimentary canal from 

 the ventral vessel to open into the dorsal one. From the level 

 of the twelfth seta3 to the oesophageal pouches the dorsal 

 vessel does not receive any segmental vessels from the gills or 

 nephridia, nor does it open directly into the heart (fig. 5). It 

 merely receives numerous branches from the gastric plexus. 

 In front of the heart it receives on each side a branch from the 

 third nephridium and the fifth setigerous sac ; a branch from 

 the oesophageal pouches ; and one from the second nephridium 

 and fourth setigerous sac. It then runs on and, piercing the 

 third diaphragm, receives a branch running on the anterior 

 face of the diaphragm from the first nephridium and third 

 setigerous sac. On reaching the second diaphragm it receives 

 a branch from the second setigerous sac, and after piercing the 

 first diaphragm receives a branch from the muscles forming 

 the buccal sheath. Thence the dorsal vessel breaks up into 

 capillaries around the buccal musculature, prostomium, and 

 otocysts. From these capillaries the ventral vessel takes its 

 origin. It gives off a small unpaired branch running in the 

 first diaphragm and to its pouches ; a paired branch arising 

 about midway between the first and second diaphragms to the 

 neural vessels and second setigerous sac ; a single small vessel 

 supplying the second diaphragm and the neural vessels ; an 

 unpaired vessel to the third diaphragm, to the neural vessels 

 in that region, and to the first nephridia ; a pair of branches 

 to the neural vessels and second nephridia ; and lastly, a pair 

 to the neural vessels and third nephridia. From this point 

 onwards the ventral vessel supplies the setigerous sacs, body- 

 wall, nephridia (if present), and gills by large segmental vessels. 

 The ventral vessel is very large and turgid in the gastric region, 

 and is surrounded by tufts of dark brown chlorogogenous tissue. 



