The Classification and Distribution of Earthworms. 251 



than probable that the anterior and posterior gut regions into which 

 these nephridia open are stoniodajuni and proctod;euni respect- 

 ively, i.e., epidermic involutions. Hence the existence of numerous 

 nephridial pores may be regarded as having been established 

 before the involution of epiblast to form the two extremities of 

 the digestive tract. A secondary connection seems more unlikely. 



It seems therefore permissible to regard these facts as 

 strengthening the justice of the view that the diffuse or 

 dysmetameric nephridia are the most ancient form of these 

 organs ; and, if so, they show a decided resemblance to the 

 excretory system of the Planarians, some of which worms 

 appear to me to represent, more nearly than any other living 

 group, the ancestors of the Oligochseta. 



This being so, I would associate together all those earth- 

 worms which have a nephridial system built upon the Platy- 

 helminth type into one group, on the assumption that the 

 character in which they agree must be a mark of affinity. 



This group will include three of Eosa's families, viz. : — 

 Perichsetidoe, Acanthodrilidse, and Eudrilid?e; and I term it — 



Group I. ACANTHODRILINLi 



Definition. — Earthivorms generally with a diffuse {dysme- 

 tameric) nephridial systemi ; always provided with atria 

 which are either tubular or lobate ; often provided with 

 penial setce. Clitellum commencing in the 'x.iith or 

 xiii^/i seg^nent, and of variable extent. Male generative 

 poi'es on xvii^A or xviii^/i segment. Spermathecoi always (T) 

 furnished with diverticula. 



This group is divisible into the following families : — 



1. Family Perichsetidae. 



Definition. — Earthworms with numerous setce per segment 

 arranged in a continuous ring, sometimes loith dorsal 

 and ventral gaps, 20 <o 100 in number. Nejohridia nearly 

 always diffuse. At7'ia lobate or (rarely) tubular ; penia 

 setce generally absent. 



1 Exception may be taken to this name, particularly as I regard the Peri- 

 chfetidffi as the typical family. I adopt it, however, for the reason that the 

 diffuse nephridia were first made known in Acanthodrilus, and that the name 

 may be taken to express the fact that the majority of its members have penial 

 setse. This led Perrier to apply the name Acanthodrilus to the genus. 

 VOL. X. S 



