Ill] EXTERNAL FEATURES 61 



genus to genus and from species to species. It is 

 sometimes further forward and sometimes further 

 back. In the remarkable family Moniligastridae this 

 organ is developed earlier in the body than in any 

 other group of true earthworms, consisting of four 

 segments or so commencing with the tenth. As a 

 rule the clitellum begins further back than this the 

 thirteenth or fourteenth being a common place for 

 the first commencement of the organ among the 

 Megascolecidae, while among the Geoscolecidae and 

 Lumbricidae it is generally much further back, 

 commencing in Alma at the forty-fifth. These 

 details might be increased to many pages ; but enough 

 has been said to emphasise the variability of the 

 organ. What reason can be assigned to this varia- 

 bility, which might be supposed unnecessary in view 

 of its functions ? There are perhaps two suggestions 

 that may be made, though many facts are lacking 

 which might offer confirmation or refutation of either 

 of these. It is to be noticed that on the whole the 

 older types such as the Moniligastridae and the 

 Megascolecidae (including for this purpose the 

 Eudrilidae) have clitella which are short. There 

 are a few but not many exceptions. These older 

 types do not seem capable of extending their range 

 with any rapidity. It is true that here again there 

 are exceptions, notably many species of PJieretima 

 which are considered under the section which deals 



