ON TWO NEW GENERA OF AQUATIC OLIGOCH^TA. 283 



The structure of this diverticulum of the sperm duct is precisely that of the spermathecse, 

 which lie in the following segment — the XHIth. It is, however, of somewhat less calibre, 

 otherwise it might have been supposed to be a second pair of spermathecse lying in 

 the XHIth segment. 



§ Atria. 



The transition between the vas deferens and the atria is quite abrupt as regards the 

 character of the epithelium lining the two tubes. But the diameter of the atrium is at 

 first exactly equal to that of the vas deferens ; it becomes gradually wider, and then 

 narrows again towards its external aperture, which is situated upon the Xllth segment. The 

 external pores of each side of the body are quite evident in the specimen, which was 

 mounted entire in Canada balsam, lying in front of the ventral pair of setae of the Xllth seg- 

 ment. One remarkable point about the atrium of Phreodrilus is its great length; but instead 

 of extending through a large number of segments, as in Sutroa [Eisen, 4], the entire atrium 

 is contained in the Xllth segment. It is, however, coiled upon itself several times, and 

 is thus able to be stowed away in one segment. The structure of the atrium is the same 

 throughout. It is illustrated in fig. 4 of Plate I. ; the atrial epithelium is apparently 

 composed of columnar cells, the boundaries of which were not visible in my preparations. 

 The individual cells could only be separated by the nuclei, which were much more darkly 

 stained than the surrounding protoplasm. The epithelium of the whole atrium was 

 thrown into folds. I could detect no trace of cilia anywhere ; and, as the cilia of the 

 vas deferens and other organs were beautifully preserved, I am disposed to think that the 

 atrium of this genus is not ciliated during life. At the external pore the atrial epithelium 

 passes without any break into the epidermis. There was no trace of a penis, or any 

 specialisation in the distal section of the atrium. As has been already remarked, the 

 only difference between the distal and middle region of the atrium is the less calibre of 

 the former. 



The distal section of the atrium, which passes obliquely backwards from the external 

 pore, in addition to its epithelial lining, is covered externally with a layer of muscular 

 fibres and a thin layer of peritoneum outside of this. The muscular fibres run in a circular 

 direction ; inside of them is a recognisable membrane by which they are separated from 

 the epithelium. The peritoneal layer which covers the muscular layer is extremely thin. 

 In sections this layer can be only detected by the nuclei, which are quite as large as the 

 nuclei of the epithelium. 



At some distance from the external orifice of the atrium the muscular and peritoneal 

 coats become widely separated from the epithelial layer. At this point the lumen of the 

 atrium becomes suddenly contracted, as is shown in fig. 30 of Plate II. The muscular 

 layer, and the peritoneum which covers it externally, becomes completely detached from 

 the epithelium, and a wide space is thus left (see fig. 30, sp., and 9, sp.), which is filled 

 with spermatozoa. Besides spermatozoa, which lie separately, and are not in any way 

 aggregated into bundles, this space contained numerous free nuclei, which appear to have 



