CHAP, vi THE EARTHWORM 327 



gardener and agriculturist, as they are continually 

 ploughing and manuring the soil, and in doing so they 

 gradually cover up stones and other objects lying on the 

 surface. 



The body of the earthworm is long and narrow, 

 approximately cylindrical in shape, and bilaterally 

 symmetrical (p. 296) : in the common forms it reaches 

 a length of about six inches. Anteriorly it is bluntly 

 pointed, while more posteriorly it is somewhat flattened, 

 its greatest diameter being reached at about a third of 

 the entire length from the anterior end. In the ordinary 

 creeping movements of the animal, which are effected 

 by the alternate contraction and extension of its body, 

 the anterior end is directed forwards. The colour is 

 pinkish in most species, and is paler on the lower or 

 ventral than oh the upper or dorsal side. 



The surface of the body is distinctly marked by trans- 

 verse annular grooves into body-segments or metameres 

 (Fig. 82), the number of which is about 150, more or 

 less : the segments are rather longer towards the anterior 

 end than they are further back. At the extreme anterior 

 end is a small finger-shaped head-lobe or prostomium, 

 which overhangs the mouth, situated on the antero- 

 ventral surface of the next segment, which is therefore 

 called the peristomium, and is counted as the first 

 metamere. The anus is a slit-like aperture on the 

 hinder surface of the last or anal segment. The earth- 

 worm is thus a metamerically segmented animal, and 

 the segments are serially homologous with one another 



(P- 39)- 



In adult worms a prominent glandular swelling is 

 noticeable on the dorsal and lateral surfaces of the body ^ 

 extending through from five to ten segments beginning 

 at about the thirtieth ; this is known as the clitellum, and, 

 as we shall see, it is important in the process of impreg- 



