AN ATTEMPT TO CLASSIFY EAETHWORMS. 275 



normally — i.e. in couples — anteriorly, but become separated 

 posteriorly, or even, in Urochseta, alternate from somite 

 to somite. That is, according to my view, modification has 

 commenced posteriorly, but has not affected the whole of the 

 body; whilst in Diachseta this change has extended all along 

 the worm.^ 



The perichaetous condition, according to my view, has arisen 

 firstly by the separation of the individual setse, originally 

 in couples, so as to produce eight equidistant setse (as in 

 species of Acanthodrilus, in Plutellus, and Allolobo- 

 phora boeckii) ; and then intermediate setse have appeared 

 gradually filling up the spaces, leading on through Deino- 

 drilus with twelve, to Perichseta with 20 — 100 per somite. 

 I conceive this intercalation of setae to be effected by the 

 gradual increase in length of the accessory setae (" soies de 

 remplacement " of Perrier), which are very usually found, one 

 to each of the functional setae in many, perhaps in all earth- 

 worms. Supposing all the accessory setae of a somite became 

 thus fully developed contemporaneously with the existent 

 setae, we should get a doubling of the setae, i. e. sixteen per 

 somite. Each of these would, later on, have an accessory 

 seta, and these might develop into functional setae, and so 

 on, till we get the perichaetous condition. 



Mr. Beddard would regard the penial setae in special sacs, 

 found in many earthworms, as vestigial representatives of a 

 perichaetous condition. I would regard them, however, as 

 secondary and as developed from ordinary accessory setae, 

 which if carried to a greater extent would lead to a peri- 

 chaetous condition. If we look upon the perichaetous condi- 

 tion, then, in this light, the removal of Perionyx from its 

 associations with Perichaeta merely indicates that the con- 

 dition has been developed twice, and independently ; and if we 



1 In a new species, D, wind lei, Beddard states that there are no setae 

 on the first five somites. Here the modification has gone further, and the 

 setae have disappeared altogether. Microchseta presents a somewhat 

 similar case of disappearance of setae and fusion of somites. This condition, 

 of course, may have resulted also from a perichaetous condition. 



