AN ATTEMPT TO CLASSIFY EARTHWORMS. 279 



same time^ in the condition of its excretory system and in 

 the matter of the dorsal blood-vessel, Moniligaster is in 

 a less primitive condition than many other worms, which, 

 whilst advancing in respect of certain of their other organs, 

 retain the primitive network of tubules more or less com- 

 pletely. 



My idea as to the relation of the various families is as fol- 

 lows: — From some of the earlier " Limicolous" forms — Lum- 

 bricoraorpha minora — the earthworms have been derived 

 along two lines. ^ Along one branch (a) the more primitive 

 plectonephric condition has been retained from some Platy- 

 helminth ancestor of the whole Chsetopoda. Along the other 

 (b) this has been replaced by the meganephric condition 

 more usually found in the group. 



The Typhseidse, having a single pair of prostates, stand 

 at the end of the main branch of the first line (a) ; but from 

 this line a branch has given rise to the Acanthodrilid£e, 

 to which Dichogaster has some affinity. 



The Perichsetidse appear to have arisen from the Typhseid 

 stem — from some form with flattened prostates, by multipli- 

 cation of setse. Deinodrilus, having a dozen setae, would 

 not necessarily be related to the Perichaetidse, but might 

 point to the possibility of the development of a perichsetous 

 condition in the family Acanthodrilidae. 



The branch (b) leads through Moniligaster to the Geo-' 

 scolecidse, which retain the single pair of testes, &c., and 

 exhibit amongst the genera stages in the separation of the 

 setse, but which have lost the prostate, a primitive character 

 of the group. Springing from this branch is another, leading, 

 after the appearance of the second pair of testes, &c., through 

 the Eudrilidse to the Perionycidse. 



The loss of prostates and the extension of the clitellum gives 

 us a new line leading to Rhinodrilidse, which, through 

 Hormogaster, presents some affinity to the Lumbricidae. 



' It is quite possible, of course, that earthworms have not been derived 

 from water-worms ; the latter may have been developed from earthworms, 

 but I think the evidence is in favour of the statement in the text. 



