58 M. E. Mecznikow on the Rhabdoceela. 
filled with ova, lies on the side of the body. Near it there is a 
pyriform uterus (wé.), which is continued into a vagina opening 
outwards. The yelk-stock is also to be seen as a long band-like 
structure; aud at the inferior side of the body there is a very 
large double receptaculum seminis (r.s.), filled with zoospermia, 
the orifice of which [ 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 of Prostomum lineare, as just 
described, does not precisely agree with 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 Prostomum 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 my 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 Claparéde 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 (Pl. 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 (fig. 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 
