58 MR. F. E. BEDDARD ON [Feb. 4, 



condition. The summary which Eisig gives of our knowledge re- 

 specting the structure and development of the parapodia and their 

 setae in other Polychseta does not permit of a decisive answer as to 

 the original condition of these organs ; the " diplostichous biremal " 

 form may have been evolved from a " monostichous uniremal," or 

 the reverse. The former alternative is more in accord than the 

 latter with the derivation from a continuous circle of setse. 



Among the Oligochseta there is more evidence ; and this seems to 

 favour the supposition that the continuous circle of setse is the 

 archaic condition. 



(1) The continuous circle of setse characterizes the genera Perichceta 

 and Perionyx ; of these the former is the most widely distributed and 

 the most abundant of all Earthworms. There are more species of 

 Perichceta than of any other two genera ; that is, of course, well 

 described species. There is, moreover, a large amount of structuial 

 variation in the species of this genus ; so much so, that were it not 

 for the fact of the agreement among the species in the very striking 

 character of the setfe, they would probably have been more subdivided 

 into genera ; this I have attempted to do. Such forms as P. inter- 

 media, P. stuarti, P. ceylonica, and P. affinis differ from each other 

 quite as much as do such genera as Ui-ochceta, Diachceta, and 

 TJrobenus. Accordingly when the existence of some 18 or 20 genera 

 possessing only 8 setse in each segment is contrasted with the two 

 genera above-named as an argument in favour of the more prevalent 

 " biramous " condition, it must be discounted b_v these considerations. 

 Even with regard to the number of species, Perichceta and Perionyx 

 are probably not far behind the remaining genera of Earthworms 

 taken together, though it is difficult to make an estimate \ 



(2) The Perichsetidae show in many respects a type of structure 

 which is less specialized and more primitive than that of other Earth- 

 worms. The continuous network of nephridia with numerous 

 irregularly disposed internal and external apertures is, so far as our 

 knowledge goes, confined to that genus and found in nearly all its 

 species. In other genera which have a diffuse nephridial system 

 {Megascolides, Typhosus, some species of Acanthodrilus, and Crypto- 

 drilus, Beinodrilus, Trigaster, and Dichogaster) there appears to 

 be generally some modification — such as loss of funnels, specialization 

 of part of nephridial network, restriction of network to segments, 

 &c. — which can be best explained on the hypothesis that it has been 

 derived from a condition like that of Perichceta. 



(3) In most ( ? all) Perichsetidse the buccal lobe does not divide the 

 buccal segment ; this appears from the nature of the case to be a 

 primitive condition. Most Perichcetce have dorsal pores, the presence 

 of which may fairly be regarded as typical for the terrestrial Oligo- 

 chseta : it is worthy of note that some forms, in which these pores are 

 absent, show signs of degeneration ; for example the absence of dorsal 

 pores in Acanthodrilus georgianus and in Pontodrilus is correlated with 



' About 60 species of Perichceta to about 120 of other genera : but the 

 differences between individual s'ltecles o\' Lumbricusatiii Allolobophora (comprising 

 00 out of llic 120) are often very small. 



