240 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



Small vessels collect the blood from these parts and 

 unite to form the ventral vessel which, soon after its origin, 

 gives off a median vessel to the first diaphragm and 

 its pouches, then a pair of vessels to the nerve cord and 

 the body wall in the neighbourhood of the second setal 

 sacs. A little further back it gives off a median vessel 

 running on the second diaphragm to the nerve cord, and 

 a similar vessel on the third diaphragm to the nerve cord 

 and first nephridium. From this point the ventral vessel, 

 as it proceeds backwards, supplies the chsetigerous sacs, 

 body wall, nephridia and gills (in those segments in which 

 the two latter are present) by segmentally arranged 

 branches. As it runs along the intestine the ventral 

 vessel is connected with the dorsal vessel by the intestinal 

 vessels (see above), and it finally breaks up into capillaries 

 near the anus. 



On the walls of. the stomach and intestine are 

 numerous intersecting blood-streams. In young specimens 

 the blood is contained in a plexus of small vessels, but as 

 the animal grows these vessels become converted into a 

 system of sinuses, and in old specimens the stomach 

 practically lies in a gastric blood sinus, which is situated 

 between the gut epithelium and the ccelomic epithelium 

 which covers it. The dorsal vessel is, however, more or 

 less distinct. 



It is convenient to refer here to two portions of the 

 plexus or sinus which are somewhat differentiated. (1) 

 There are two vessels (or sinuses) ventrally situated, and 

 known as the sub-intestinal vessels (figs. 28, 36). They 

 commence just behind the heart, and may be traced back- 

 wards to the level of the twelfth setas, behind which point 

 they gradually disappear. They receive efferent vessels 

 from the first six pairs of gills. (2) On each side of the 

 anterior part of the stomach there is a lateral gastric 



