A REN ICO LA, 



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 to a 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, 



FIG. 115. Arenicola marina. 



Entire animal viewed slightly from left side. Note anterior mouth ; 

 setae on anterior region ; setae and gills on median region ; 

 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 1 6 inches, and at its thickest part is about 

 half an inch in diameter. There are three regions in the 

 body : (a) The anterior seven segments, of which all but 

 the first have bristles ; (b) 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. 



