SEA-FISHERIES LABORATORY. 237 



pharynx, tends to produce a thorough, mixing- of the sand 

 and the digestive substances, and in this way the food, 

 which consists of organic substances of various kinds in 

 the sand, is brought into contact with the digestive secre- 

 tions. The ciliary action of the lateral and ventral 

 grooves probably separates the digested substances from 

 the sand, and carries them slowly backwards. The ciliated 

 grooves are in close association with the blood sinuses in 

 which the How of blood is probably slowly forwards. The 

 ventral groove is in especially close relation to the sub- 

 intestinal vessels (fig. 36). It seems probable, there- 

 fore, that the blood in the gastric plexus absorbs the 

 nutrient materials, conveys them to the hearts which 

 pump the blood along the ventral vessel to various parts 

 of the body. 



A thin cord of mucus from the ventral groove may 

 often be seen in freshly-formed castings. 



The great abundance of Arenicola wherever the sand 

 or mud contains a large proportion of decomposing matter 

 or sewage, and its absence from or scarcity in long- 

 stretches of coast where the beach is formed of clean 

 sand, indicate that these animals feed on decaying animal 

 or vegetable matter. They remove some of the decom- 

 posing organic matter in the sand, and as they burrow to 

 a depth of two feet or more, they cleanse the sand to 

 about this depth, and discharge it on the surface in a 

 purer condition. (See also under Economic section.) 



Burrowixg. 



The burrowing of Arenicola is performed by the 

 combined action of the proboscis, the swollen anterior 

 region of the body, and the waves of muscular contraction 

 which pass along the body from behind forwards. The 



