1885.] Nephridia of a New Species of Earthworm. 463 



outermost of the two setae which compose the pair ; in the genus 

 Titanus, where the nephridial orifices are dorsal in position, they open 

 close to the outermost of the two dorsal nephridia. There thus appears 

 to be a constant relation between the nephridial apertures of both the 

 dorsal and ventral series to the outermost seta of each pair. His 

 investigations into the anatomy of Plutellus led M. Perrier to abandon 

 this position. In Plutellus the setae are arranged, as in the genera just 

 referred to, in eight longitudinal rows, and it has been already stated 

 that the nephridia themselves alternate from segment to segment, 

 sometimes opening by one of the dorsal, at other times by one of the 

 ventral setae ; there is, however, no constancy to any one in particular 

 of the two setae which compose the pair ; occasionally the nephridial 

 orifice will be found close to the outermost, and occasionally close to 

 the innermost, of the two setae of its pair. These facts are clearly 

 not explicable by assuming the typical presence of two series of 

 nephridia in earthworms, bnt they become perfectly clear and 

 intelligible in the light of the facts that I have been able to bring 

 forward in the present paper ; in Plutellus there are the remains not 

 only of two series of nephridia, bnt of four, each corresponding to one 

 of the four rows of setae of each half of the body. 



We are therefore now in a position to extend Professor Lankester's 

 hypothesis, and to assume that to each seta, and not to each pair of setce, 

 corresponds a separate nepliridium. ~No donbt Professor Lankester's 

 hypothesis is correct so far as it goes ; it is very probable that when 

 the two setae of each pair come to be placed close together, one of the 

 two nephridia disappears. 



Another question raised by the foregoing facts concerns the 

 general problems relating to the distribntion of" the setae in earth- 

 worms ; are we tc regard the presence of four series of pairs of setae 

 as typical for earthworms, or is it possible that this condition has 

 been arrived at by a process of rednction, the primitive condition 

 being a complete ring of setae ronnd each segment, as is actually 

 f jund in Perichaita ? 



There are a good many facts which appear to support the former 

 alternative; (1) the undoubtedly close resemblance between the two 

 pairs of setae of an earthworm, and the dorsal and ventral parapodia 

 of a Polychaetous worm ; (2) the fact that in the young Perichceta the 

 setae are by no means so numerous as in the adult; we owe this 

 observation to M. Perrier, but unfortunately he has not recorded the 

 exact number of setae, and whether there was any real approximation 

 to the quadriserial disposition. 



Other facts point to the latter alternative. In Urochceta the setae 

 are disposed in eight longitudinal rows, bnt in the posterior part of 

 the body the setae do not exactly correspond in position in a series of 

 segments ; there is in fact a quincuncial arrangement ; in the interval 



