60 EARTHWORMS AND THEIR ALLIES [ch. 



first place and when present the numbers and 

 situation of the so-called genital papillae are among 

 the most obvious. The setae and their position we 

 treat of under the heading of the modification of the 

 worms to lead a terrestrial life ; and though these 

 chitinous organs differ greatly they do not concern us in 

 the present section. The girdle or clitellum ('eminentia 

 quasi ulcerata') has been long observed as a character 

 of these animals and it is one which distinguishes 

 them from all other worms except the leeches and 

 a very few marine Polychaeta. This modified region 

 of the body is often of a different colour to the rest 

 and has a glandular look which readily enables one 

 to recognise its position and limits, though its 

 obviousness is less in some cases. It either forms 

 a complete ring round the body or is developed upon 

 the dorsal surface and only to a slight extent upon 

 the ventral surface. Its use, as is well known, is to 

 secrete the cocoon in which the eggs are deposited ; 

 and the epidermis which forms it is thickened and 

 more glandular than that in other regions of the 

 body. Among earthworms it is doubtful whether the 

 clitellum ever occupies less than three segments; it 

 consists of three only in the great majority of species 

 of the marked genus Pheretima. From this lowest 

 level it extends in other forms, and in the partially 

 aquatic African genus Alma it may occupy as many 

 as forty segments. The position also varies from 



