58 M. E. Mecznikow on the Rhabdocoela. 



filled with ova, lies on the side of the body. Near it there is a 

 pyriform uterus [ut.), which is continued into a vagina opening 

 outwards. The yelk-stock is also to be seen as a long band-like 

 structure; and at the inferior side of the body there is a very 

 large double receptaculum seminis (r.s.), filled with zoospermia, 

 the orifice of which I could not detect. Finally, the female in- 

 dividuals also possess a poison-gland, the efferent duct of which 

 is combined with the spinous apparatus. 



The organization of the genitalia oi Prostomum /j'neare, asjust 

 described, does not precisely agree vtith the descriptions of these 

 objects cited above. In the first place must be mentioned the 

 difference in the distribution of the male and female organs in 

 the same individual, both kinds of organs being represented as 

 quite equally developed in one specimen in the figures of the 

 above-mentioned writers — a circumstance which may probably 

 be due to their having made their drawings from the observa- 

 tion of several (male and female) specimens. The second and 

 more important difference between my description and those of 

 Schmidt and Schultze is due to the fact that those savants re- 

 garded the uterus as the egg-shell, and therefore furnished the 

 egg with a peculiar stalk (Schmidt), or with a still more peculiar 

 micropyle (Schultze). In consequence of this misconception the 

 above-mentioned authors have described Frostomum lineare as 

 monoporous, and have not recognized it as an animal furnished 

 with two genital apertures, which it really is. The other, less 

 important differences between ray description and that of the 

 other observers may be seen by a comparison of the figures. 



From the preceding statements it is clear that Prostomum 

 lineare presents in a less degree the same phenomenon of inci- 

 pient hermaphroditism which Claparede observed in Convoluta. 



The peculiarities in the structure of the sexual organs of 

 Prostomum lineare are by no means common to the whole of its 

 genus, and do not even extend to the most nearly allied species. 

 This is shown by a new marine species, also provided with a 

 spinous apparatus, which I discovered in Heligoland, and there- 

 fore indicate as P. helgolandicum. The specific characters of 

 this species (PI. VIII. fig. 3), which is oval and furnished with 

 comparatively large eye-points and cerebral ganglia, relate chiefly 

 to the structure of the sexual organs. These are not so unequally 

 distributed as in the previously described species ; P. helgo- 

 landicum is perfectly hermaphrodite. The ovaries and yelk- 

 stocks (tig. 3 ov. & vit.) are paired organs running along the two 

 sides of the body ; and besides these, we may distinguish a 

 uterus {ut.) with a crown-like inner margin. Of the male 

 organs I was able to observe the two symmetrically arranged 

 seminal vesicles [v.s.) and the unpaired thick-walled vesicle 



