OCR Text |
Show 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. i. 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 Discocelis, 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. 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 shape of the accessory vesicle in that genus. Text-fig. 53 " Brain-eyes " of Semonia penangensis. |