CHAP. V THE EARTHWORM 319 



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, ap- 

 proximately cylindrical in shape, and bilaterally symmetrical 

 (p. 291) : 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 an- 

 terior end is directed forwards. The colour is pinkish in 

 most species, and is paler on the lower or ventral than 

 on the upper or dorsal side. 



The surface of the body is distinctly marked by trans- 

 verse annular grooves into body-segments or metameres 

 (Fig. 79), 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 peri- 

 stomium, 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 earthworm is thus a metamerically seg- 

 mented 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 about six segments beginning at 

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

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



