20 EARTHWORMS AND THEIR ALLIES [ch. 



nephridia are either paired or numerous and these 

 various characters allow of the sub-family being split 

 up into sixteen genera or thereabouts. 



As an example of another type of organisation 

 and as contrasting with Notiodrilus we may now 

 briefly describe the structure of the genus Ponto- 

 scolex (better known as Urochaeta), a member of the 

 American and African family Geoscolecidae. 



The worm is some four inches long and composed 

 of as many as 212 segments. Each of these except 

 the first has eight setae which for the first few 

 segments of their occurrence are disposed in four 

 pairs in the usual fashion. Behind this point how- 

 ever the setae cease to present this symmetrical 

 arrangement and are irregularly disposed so that a 

 given seta is not in the same line with the corre- 

 sponding seta of the segments in front or behind. 

 There is thus no region of the body which has 

 not a seta implanted upon it ; and the efiect is 

 therefore comparable to the condition obtaining in 

 those worms, such as Pheretima, where circles of 

 numerous setae are met with. There are however 

 only eight in a given segment. The clitellum extends 

 from segment xv to xxii or xxiii and is developed 

 only dorsaUy and laterally. It has setae like the rest 

 of the body ; but those upon the clitellum are longer 

 and more markedly ornamented than are those of the 

 body generally. The latter are not only sculptured 



