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Show 58 MR. F. E. BEDDARD ON [Feb. 4, condition. The summary which Eisig gives of our knowledge respecting the structure and development of the parapodia and their setae in other Polychaeta 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 Oligochaeta there is more evidence ; and this seems to favour the supposition that the continuous circle of setae is the archaic condition. (1) The continuous circle of setae 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 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 setae, they would probably have been more subdivided into genera ; this I have attempted to do. Such forms as P. intermedia, P. stuarti, P. ceylonica, and P. affinis differ from each other quite as much as do such genera as Urochceta, Diachceta, and Urobenus. Accordingly when the existence of some 18 or 20 genera possessing only 8 setae 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 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 estimate1. (2) The Perichaetidae show in many respects a type of structure which is less specialized and more primitive than that of other Earthworms. 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, Typhceus, some species of Acanthodrilus, and Crypto-drihts, Deinodrilus, Trigaster, and Dichogaster) there appears 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) Perichaetidae 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 Olio-o-chaeta : 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 1 About 60 species of Perichceta to about 120 of other genera; but the differences between individual species of Lumbricus and Allolobophora (comprising o0 out of the 120) are often very small. |
