ARENICOLA. 195 



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, habits, and sexual maturity) from those 

 which occur in the Laminarian zone, which is only un- 

 covered at low spring-tides. 



Ehlers states that at certain seasons the adults swim 

 about freely, but this requires corroboration. The young 

 stages are for a time pelagic. 



External appearance. — The lob-worm varies in length 



Fig. 85. — 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. 



from 8 in. to a foot, 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. 



Skin, muscles, and appendages. — Each segment is 

 marked by about four superficial rings. The epidermis 

 is pigmented and secretes mucus, and is divided into 



