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



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

 specting the structure and development of the parapodia and their 

 setse in other Polycheela 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 setae. 



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 circleof setse characterizes the genera Perich<pta 

 and Perionyx ; of these llie 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 structural 

 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 setse, they would probably have been more subdivided 

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

 media, P. stuarti, P. cei/lonica, and P. ajfinis differ from each other 

 quite as much as do such genera as Urochceta, Diachceta, and 

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

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

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

 " biramous " condition, it must be discounted by 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 estimated 



(2) The Perichsetidse 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 

 {Meyascolides, Typliceus, some species of Acanthodrilus, and Crypto- 

 clrilus, Deinodrilus, Trigaster, and Dicliogaster) 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- 

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

 abs^eut, show signs of degeneration ; for example the absence of dorsal 

 pores in Acanthodrilus (/eo7-ffianiis and in Pontodrilus is correlated with 



^ About CO species of Pi'ricJicefa to about 120 of other genera ; but the 

 differences bet ween individual species of Lnmbricus and Allolobophora (comprising 

 M out of the 120) are often very small. 



