1903. ] POLYCLADS OF THE ‘‘SKEAT EXPEDITION.” 309 
ovaries lie on the dorsal side of the body (see von Plehn [7]), 
whilst in the present species the position of the testes is normal. 
z.e. on the ventral side. 
In one or two respects this species approaches the closely allied 
Discocelis tigrina, and is perhaps intermediate between that species 
and S. maculata. But the latter species is also certainly closely 
allied to Diseocelis, and I am rather surprised to find that von 
Plehn has not instituted any comparison between the two genera. 
S. penangensis differs from S. maculata, and approaches Discocelis 
tigrina, not only in the features already mentioned, viz., the 
arrangement of the eye-spots and the position of the testes, but 
also in that the uteri unite to enter the vagina by a short common 
duct, and in having a very blunt, almost square penis. 
Text-fig. 53. 
“ Brain-eyes ” of Semonia penangensis. 
It differs from Discocelis, and resembles S. maculata, in being 
without the characteristic large prostatic cells which occur in the 
epithelium of the penis and of the antrum, and without the 
remarkable paired structures which run forward from the accessory 
vesicle of Discocelis tigrina. Further, like S. maculata, it possesses 
a definite vesicula seminalis, which appears to be absent in 
Discocelis. 
Lastly, S. penangensis is without the curious glandular vesicles 
found along the anterior ends of the vasa deferentia in S. maculata; 
and the vagina is prolonged, beyond the point where it receives 
the openings of the uteri, into a very small accessory vesicle—so 
small, in fact, that it almost escaped observation. This may be 
due to the fact that the only specimen available for section-cutting 
was not quite mature; or more probably, since the uteri were full 
of apparently ripe eggs, that this organ is undergoing degeneration. 
It is obvious that the distinction “between Discocelis and Semonia 
is a very slender one, but still, I think, sufficient to warrant the 
retention of the latter as a valid genus. The most important 
characters separating the two genera are the absence of a vesicula 
seminalis in Discocelis, and the curious horseshoe shi ape of the 
accessory vesicle in that genus. 
