120 DR. J. STEPHENSON ON THE MORPHOLOGY, CLASSIFICATION, 



Once more, Spenceriellct possesses the perichsetine arrangement 

 of setae, micronepliridia, and tubular prostates, differing only 

 in the latter respect from Megascolex, which has the more ad- 

 vanced racemose form of the glands. But Michaelsen has lately 

 (16) transferred two species of Spenceriella to Megascolex ; though 

 the branching of the central canal was not to be inferred from 

 anything in the external form of the glands, it was found to exist 

 in a slight degree on microscopical examination of sections. It 

 is scarcely rash to look on these apparently transitional species 

 as descended from species of Spenceriella, which they so much 

 resemble. They can hardly be descended from either Notoscolex or 

 Perionyx ; the transitional species in these cases are characterized 

 by the incomplete setal rings, or by the incompletely broken up 

 nephridia, and have, apparently, the fully developed racemose 

 prostates, as is usual in Notoscolex and Perionyx. 



In other words, species which anatomically belong to the 

 same genus, Megascolex, have arisen from two less specialised 

 genera, Notoscolex and Perionyx, and at least at three separate 

 times; quite possibly Spenceriella is the origin of certain other 

 species. Probably, of course, this much understates the truth ; 

 it is only a few small groups of species of Megascolex that we can 

 thus trace back at present ; the great bulk of species have 

 probably originated at still other times and in still other places. 



Michaelsen, having before his eyes the separate origin of 

 Megascolex from Notoscolex in New Zealand and Ceylon, gets over 

 the polyphyletic difficulty by merging the two genera into one (16). 

 But this is too short a way with the difficulty ; if, wherever 

 we find a polyphyletic origin, we merge the genera concerned, 

 then of course no genus will be polyphyletic, and orthodoxy will 

 triumph. And it may be noted that even this device of fusion 

 is not effective where a genus has a double origin from two 

 other genera. Assuming that some species of Megascolex have 

 arisen from Notoscolex, others from Perionyx, the fusion of 

 Notoscolex, Megascolex, and Perionyx into a single genus leaves us 

 where we were, since the genus now has a double origin from 

 Megascolides and Diporockceta (cf. text-fig. 1). 



Take now the case of Pontodrilus. From its ancestor Plutellus 

 it differs in two primary respects — the gizzard has become 

 vestigial, and there are no nephridia at all in the first twelve 

 segments ; it is littoral in habit, and is very widely distributed 

 in the warmer regions of the globe. Benham in 1903 (3) 

 discovered in a lake in New Zealand a worm with the above 

 anatomical characters, which he called Plutellus lacustris, 

 on the ground that the features wherein this worm 

 agreed with Pontodrilus and differed from Phttellus appeared to 

 be adaptive and related to an aquatic habitat ; he implies, though 

 he does not expressly state, that this worm had an origin from 

 Plutellus independent of that of the bulk of the species of 

 Pontodrilus, and cannot therefore be united with them in the 



