AN ATTEMPT TO CLASSIFY EARTHWOEMS. 273 



VI. Phylogeny. 



I will now endeavour to trace the phylogeny of the group of 

 earthworms, but owing to the scanty information as to their 

 ontogeny, it is impossible to found anything like a true phylo- 

 genetic tree. 



First of all it will be desirable to say a few words as to 

 what may be considered " primitive characters/' as two widely 

 different families have been regarded as the more primitive, 

 viz. Perichseta by Beddard, and Acanthodrilus by Rosa. 

 I hope to be able to bring forward sufficient reason for denying 

 to either of them an archaic condition. 



The excretory system, the setpe, clitellum, prostate, 

 and sperm-ducts may be taken as the more important 

 characters. 



The Excretory System. — The recent researches of Bed- 

 dard and Spencer have resulted in the conclusion that the net- 

 work of tubules is a more primitive state than the large 

 nephridia ; that, in fact, the latter have been derived in some 

 way from the former. 



In Megascolides the excretory system in the anterior 

 region of the body consists in a network of delicate tubules, 

 with numerous external apertures, but without coelomic funnels. 

 Further back, one of these tubules on each side increases in 

 size, and the network diminishes in extent; whilst in the 

 somites quite posteriorly there is on each side a large tubule, 

 which possesses a coelomic funnel, and which still retains its 

 connection with the network. Spencer regards the anterior 

 plectonephric condition as more primitive, and differs from 

 Beddard in considering the nephridial funnels as new struc- 

 tures, and not as derivatives of the flame-cells of Platyhelmia. 

 It is to be noticed that the modification begins in the posterior 

 somites, whilst the anterior part of the body still retains a 

 primitive condition. 



Other instances of the co-existence of large nephridia with 

 the network of tubules have already been given. 



