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



canal throughout a considerable region of the body. In the 

 Oligocheeta, on the contrary, this proliferation is restricted, 

 and the pi'oduction of sexual cells is confined, in genera], to one 

 or two testicular segments and a single ovarian segment in the 

 anterior part of the body ; the gonads, moreover, arise from a 

 narrowly circumscribed area on the posterior face of the septa. 

 It is probable that the OligochiBta have originated from some 

 group of Polychfeta ; the evolution of the gonads has therefore 

 been from a non - localized to a more definitely localized 

 condition. 



In Chcetogaster orientalis the gonads (the male, at least) have 

 again lost their localized character; they are no longer definitely 

 circumscribed prolifei'ations of peritoneal cells in a constant 

 position. Tha,t this condition, which is not unlike that of the 

 Polychfeta, i-epresents a regression, and not the persistence of a 

 primitive state, may be taken as certain ; Chcetogaster is the 

 most modified genus of the Naididfe — of the numerous characters 

 in which it difiers from the rest of the fainilj^, not one can be 

 thought of as primitive ; and I believe the Naididee to be 

 themselves far from primitive among the Oligochpeta. 



We may for the sake of clearness express the facts in a 

 somewhat diflferent way. In certain animals the first visible 

 dift'erentiation in development is that of the germ-cells ; in 

 Ascaris, for example, as is well known, the peculiarities of theii- 

 chromatin constitution enable them to be distinguished from the 

 first. And it is possible that a similar "germ-track" exists in 

 all animals, but that we are imable, with our present means of 

 investigation, to distinguish it in all — at least in its earlier 

 stages. It is possible, in other wox-ds, that in all animals the 

 germinal material (germ-plasm) is set apart from the first in 

 certain cells and their descendants, and that in the shaping of 

 the organism these find their way, sometimes by fairly extensive 

 wanderings, to their station in the gonads, or rather to their 

 particular station in the coelomic lining membrane (in the 

 Triploblastica) where, by their proliferation, they give rise to 

 the gonads. In the Polycheeta their distribution is wide ; in 

 the Oligochfeta extremely circumscribed ; in Chcetogaster orien- 

 talis it has again become wide. 



With what peculiarity of structure or habit, or Avith what 

 feature of the past history of Chcetogccster can this change be 

 correlated ? I can think of only one that is at all likely, and 

 even here I do not think it is easy to suggest how the change 

 has been brought about. 



I have previously suggested (5) that Chcetogaster is descended 

 from parasitic ancestors. I do not wish to repeat the argument 

 at length ; I will only say that Chcetogaster, alone of the Naidida?, 

 never shows the phenomenon of an ascending current of water 

 beginning at the anus and impelled up the intestine by anti- 

 peristalsis and ascending ciliary action ; and that the loss of cilia 

 and of the ciliary function of the intestine is presumably due 



