ON TWO NEW GENERA OF AQUATIC OLIGOCHSETA. 287 



The numerous free nuclei which lie in the circumatrial sac are very remarkable. They 

 agree very closely in general appearance with the nuclei of the peritoneal cells which 

 cover the sac externally ; but I have not found any such a layer of nuclei lining the 

 internal wall of the sac. Although the nuclei lie among the spermatozoa, it has not 

 appeared to me that there is any connection between the two. And as the spermatozoa, 

 which I found abundantly in all stages of development in the Xth and Xlth segments, 

 develop in the usual way in which the spermatozoa of Earthworms and other Oligochseta 

 have been shown to develop, I cannot see how any such nuclei can be traced to the 

 germinal cells. Moreover, I could detect no such nuclei among the sperm polyplasts so 

 abundantly present in segments X and XI. 



Another noteworthy point about the atrium of this Annelid is the total absence of 

 cilia from its lining epithelium ; the vas deferens is of course ciliated, but not the atrium 

 or the appendix of the vas deferens. The ciliation of the atrium is so constant a feature 

 of the lower Oligochseta that it is remarkable to find an exception to the rule in a form 

 like Phreodrilus, which perhaps comes nearer to the Naidomorpha than to any other group. 



§ Ovaries. 



These gonads (fig. 5, ov.) are paired, and arise from the intersegmental septum between 

 segments XI/XII in a position corresponding to that of the testes ; they lie therefore in the 

 Xllth segment below the funnel of the vas deferens ; but they do not also extend into the 

 segment in front as the testes do. The ovaries are limited to the Xllth segment. 



I have been able to observe certain stages in the development of the ova, which shows 

 a remarkable parallelism to the development of the spermatozoa. 



Towards the attachment of the ovary, the cell outlines were indistinguishable, and the 

 nuclei alone indicated the separate cells ; the rest of the ovary was made up of spherical 

 groups of cells presenting the structure shown in fig. 37, a ; each of the cells is pear- 

 shaped, the nucleus being in every case embedded in the wide end of the cell ; the 

 apices of the cells nearly meet in the centre of each sphere, where there is a minute 

 portion of non-nucleated protoplasm, more plainly to be seen in the later stages of develop- 

 ment. These spherules become detached from the ovary, and undergo their further 

 development in the body cavity. 



Many clumps of developing ova were to be seen lying in various parts of the ccelom of 

 segment XII. I found others (not so many) in segment XI among the developing sper- 

 matozoa. This may possibly be due to the presence of an additional pair of ovaries belonging 

 to the Xlth segment, but I have no other evidence which points to such a conclusion. 



There was no trace of any egg sac other than a pushing out of the intersegmental sep- 

 tum between segments XII and XIII, and this is illustrated in fig. 31. Later, it may be 

 that this pushing out of the septum results in the formation of a nearly closed egg sac. 

 But in my specimen it opened by a very wide mouth into the cavity of segment XII. 

 The interior of this sac was nearly full of groups of developing ova. 



