258 PHYLUM ANNELIDA 



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 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 : (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 

 numerous polygonal areas, separated by shallow grooves. 

 Beneath the epidermis is a sheath of circular muscles, and 

 then a layer of longitudinal muscles. Besides these there 

 are (from the middle of the gullet to the beginning of the 

 tail) thin oblique muscles arising from the sides of the 

 nerve-cord, and dividing the body cavity longitudinally into 

 a central and two lateral compartments. Other muscles 

 control the prostomium, the proboscis, and the bristles. 

 Unlike many of the marine Annelids, Arenicola has very 

 rudimentary appendages. This reduction of appendages 

 must be associated with the animal's mode of life ; it 

 occurs also in many tube-inhabiting worms. Neither the 

 prostomium nor the first segment shows any trace of 

 appendages, but the next nineteen have rudiments. The 

 dorsal part (notopodial) consists of a tuft of bristles, whose 

 bases are enclosed in a sac ; — the ventral part (neuropodial), 

 separated by a short interval, bears several hooks. 



Nervous system. — This is in its general features like 

 that of the earthworm, but ganglia are not developed. In 

 the ventral nerve-cord, the ring round the gullet, and 

 the slight cerebral enlargement which represents a brain, 



