HABITS AND STRCTCTURE OF ARENICOLA MARTNA. 15 



organic substances^ in the sand, is brought into contact with 

 the digestive secretion. The ciliary action of the lateral and 

 ventral grooves probably separates the digested substances 

 from the sand and carries them slowly downwards and back- 

 wards. The lining of the stomach is very thin, and the lateral 

 and ventral grooves are in specially close contact with the 

 blood-plexus, in which the flow is, probably, slowly forwards, 

 more rapidly in the sub-intestinal vessels. It seems probable, 

 therefore, that the blood in the visceral plexus conveys the 

 nutritive material to the hearts, which pump it along the 

 ventral vessel to the various parts of the body. 



The action of the chlorogogenous tissue round the stomach, 

 and particularly of that in the neighbourhood of the ventral 

 vessel and its branches, is uncertain. 



6. Vascular System (PI. 2, fig. 5). 



The blood-vascular system of Arenicola attains a high 

 degree of perfection. The large size of the chief vessels, 

 the great development of the capillary system (especially 

 on the walls of the alimentary canal), and the mechanism 

 for promoting the flow of the blood, are features that dis- 

 tinguish it. 



There are two chief vessels running, one above, and the 

 other below, the alimentary tract from end to end, — the dorsal 

 vessel, which contracts fairly rhythmically from behind for- 

 wards ; and the ventral vessel, which is feebly, if at all, con- 

 tractile. The walls of the gastric and intestinal portions of 

 the gut areenclosed in a blood-plexus, and the oesophageal region 

 is supplied by lateral vessels. The gastric vessels are connected 

 with the ventral vessel by a pair of " hearts " placed a short dis- 

 tance behind the oesophageal pouches (fig. 5, V.). These hearts 

 drive the blood from the gastric vessels into the ventral vessel. 



The dorsal vessel {D V) arises near the anus, and as it runs 

 along the intestine gives off" in each somite a pair of branches 



' Saint Joseph fouud in an Arenicola a wiiole Nereis almost digested. 

 ' Ann. Sci. Nat.,' series vii, t. xvii, 1894, p. 127. 



