274 W. B. BENHAM. 



In Perichfeta we have certainly a primitive condition, but 

 more modified than in Megascolides, in that, at any rate 

 in some species, the plectonephric tubules are provided with 

 funnels, and in others co-exist with large nephridia.^ 



2. The Setae. — Beddard considers the perichaetous condition 

 as antecedent to the octochaetous. Now, I believe we have 

 ample evidence that the reverse is the case. Firstly, it is a 

 nearly universal character of the Chaetopoda that the setae are 

 in two bundles on each side of each somite ; in the Polychseta 

 there are many setae in each group, in the Oligochaeta only a 

 few, and in a very large number of cases only two. In the 

 Archi-annelida setae may be absent or only in one bundle on 

 each side in each segment, but it is not unlikely that this 

 group contains degenerate and not primitive forms. 



In Perichaeta itself it is very usual to find fewer setae on 

 the anterior somites than posteriorly. Unfortunately, as far 

 as I am aware, we are not in possession of actual details as to 

 the mode of development of the setae in this genus. But if 

 the modification of nephridia in Megascolides commences 

 posteriorly and works forward, may we not assume that the 

 same has happened in the case of the setae of Perichaeta or 

 Perionyx? If this were so, we should expect to find just 

 what is actually existent, fewer setae anteriorly, i. e. less modi- 

 fication than posteriorly where greater modification has taken 

 place. 



In some of the species (P. attenuata and P. enormis) 

 described by Fletcher (' Proc. Linn. Soc. N.S.W.,' vol. v, 1888) 

 there are only eight setae in four couples in the first few 

 somites ; then twelve in some of the following somites ; and 

 posteriorly they become more numerous. In P. dorsalis, 

 only 16 at first, more posteriorly 30. In P. monticolla, only 

 16 per somite on first few rings, increasing to 27 about 

 clitellar region, and behind to 50. 



Agaiu,inUrochaetaand in Geoscolex the setaeare arranged 



' For a discussion of the subject see Baldwin Spencer's monograph on 

 Megascolides, and Beddard's papers in ' Quart. Journ. Micr. Sci.,' xxviii 

 and xxix. 



