ARENICOLA. 223 
sand for the sake of the organic particles and small organisms 
which it contains. The sandy castings, which pass from 
the end of the food canal, and are got rid of at the mouth 
of the tube, fall into spiral coils. It has been calculated 
that in a year the average volume of sand per acre thus 
brought up in castings is about 1900 tons, representing a 
layer of 13 in. spread out over the surface. This work, 
comparable to that of earthworms, tends to cleanse the sand 
and to reduce it toa finer powder. When getting rid of the 
casting, the worm lies with its tail upwards and its head 
downwards, or with its body bent like a bow; when the 
tide comes in, the mouth may protrude at the other end of 
the U-shaped tube. The worms that live between tide- 
marks seem to differ in many respects (as to colour, gills, 
Fic. 115.—<Aventcola marina. 
Entire animal viewed slightly from left side. Note anterior mouth; 
sete on anterior region; sete and gills on median region 3 
thinner tail region often longer than shown. 
habits, and sexual maturity) from those which occur in the 
Laminarian zone, which is only uncovered at low spring-tides. 
External appearance.—-The lob-worm varies in length 
from 8 to 16 inches, and at its thickest part is about 
half an inch in diameter. There are three regions in the 
body: (2) The anterior seven segments, of which all but 
the first have bristles; (4) the middle region of thirteen 
segments, with both gills and bristles; (c) the thinner 
posterior part of variable length, without either gills or 
bristles, and with an inconstant number of segments (up 
to about thirty). In the very front there is a head-lobe or 
prostomium, but there are no tentacles or eyes. Anteriorly 
a soft proboscis is often protruded from the gut. The 
anus is terminal. 
