THE HABITS AND STRUCTURE OF ARENICOLA MARINA. 99 



fluid neutral. In the stomach several changes occur. The secretion 

 of the gastric cells proper is probably digestive, and this, together 

 with a further amount of mucus, is mixed with the sand, and 

 shaken together by the swing of the loose gastric loop. In 

 this way the food, which apparently consists of the organic sub- 

 stances 1 in the sand is brought into contact with the digestive 

 secretion. The ciliary action of the lateral and ventral grooves probably 

 separates the digestive substances from the sand and carries them slowly 

 downwards and backwards. 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 (PL VII. Fig. 5). 



The blood-vascular system of Arenicola attains a high degree of per- 

 fection. 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 distinguish it. 



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

 the alimentary tract from end to end, — the dorsal vessel, which con- 

 tracts fairly rhythmically from behind forwards ; and the ventral vessel, 

 which is feebly, if at all, contractile. The walls of the gastric and 

 intestinal portions of the gut are enclosed 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 distance behind the oesophageal pouches (Fig. 5. V.). These 

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



The dorsal vessel (DV) arises near the anus, and as it runs along the 



1 Saint Joseph found in an Arenicola a whole Nereis almost digested. 

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



