157 
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 flow of blood is probably slowly forwards. The 
ventral groove is in especially close relation to the sub- 
intestinal vessels (fig. 56). 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 Arenzcola 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 Hconomice section.) 
BURROWING. 
The burrowing of Avenicola 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 
