ON TWO NEW GENERA OF AQUATIC OLIGOCH^TA 295 



distinguished by the thickened epithelium, developed only on the dorsal side, which 

 begins and ends abruptly. A few muscles attached to the pharynx connect it with the 

 body wall. The oesophagus is narrow, but the commencement of the intestine is hardly 

 wider. The latter is distinguished by its epithelium being ciliated. 



The chloragogen cells commence in the Vth segment. It was Claparede who first 

 noticed that the commencement of the chloragogen layer covering the intestine was a 

 fixed point often characteristic of the species. 



§ Nephridia. 



The nephridia, instead of being, as is the rule in the aquatic Annelids, absent in the 

 genital segments, are present in all the segments of the body, commencing with the Vllth 

 and excepting the Xlth and Xllth. 



At present the only instance of an Oligochset, included by Claparede in his group 

 " Limicolse," where the nephridia are not absent from the genital segments, is Lumbri- 

 culus. This fact has been recently discovered by Vejdovsky, who has, however, only 

 stated that the nephridia persist in the spermathecal segments. A fuller account of this 



v 



form is promised by Dr A. Stolc. Now that two genera are known in which the 

 nephridia persist in the genital segments after the worm has attained sexual maturity, it 

 is obviously impossible any longer to retain a group " Limicolse," distinguished by the 

 absence of nephridia in those segments. Vejdovsky suggested that the disappearance 

 of the nephridia in the genital segments of aquatic Oligochseta on the development of the 

 sexual organs and ducts might be due simply to want of space in these small Annelids. 

 In support of this suggestion it may be noted that both Lumbriculus and Pelodrilus 

 are large worms as compared with the majority of the aquatic forms ; but as they are 

 equalled or exceeded in size by many Lumbriculidse, and even by certain Tubificidse, and 

 a species of Pachydrilus, some other cause must be sought for the comparatively rare 

 persistence of the nephridia in the aquatic Oligochseta, and their almost universal persist- 

 ence in the generative segments of the terrestrial forms. 



I should regard it myself as simply a further illustration of the structural simplifica- 

 tion which is so frequently associated with smallness of bulk, and which Dohrn and 

 Lankester regard as degeneration. On this view we should expect to meet with this 

 simplification of structure less marked in those forms which approach nearest to the 

 higher Oligochseta. And that is precisely the position occupied by the Lumbriculidse and 

 Phreoryctidse, in the neighbourhood of which families Pelodrilus should undoubtedly be 

 placed. 



A very convenient method of studying the anatomy of this worm, which I found too 

 small to dissect, is to cut the anterior end of the body in half by a longitudinal cut ; the 

 ventral and dorsal halves are then mounted in glycerine, and the relative position of the 

 organs, as well as to a certain extent their minute structure, may then be very easily 

 studied. This forms a good way of checking the results obtained by continuous series 

 of longitudinal sections. 



