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Show 1 9 0 3 . ] POLYCLADS OF THE " SKEAT EXPEDITION." 311 consists of a very thin layer of circular muscle-fibres, lined with a cubical epithelium, the nuclei of the cells of which lie close to the lumen, and which has rather the appearance of an exhausted secretory tissue. This organ I call the prostate (PI. XXIII. fig. 4, pro.). About halfway between the penis and female aperture lies an organ which bears some resemblance to the penis already described. This organ constitutes one of the most interesting features of this anomalous species. It consists of a small pro-trusible penis-like organ (p. ?), about one-third the size of that first described, lying in a small antrum. It is pierced by a short duct, which runs into it from a small non-muscular vesicle which lies immediately above it (pr. ?). This small vesicle is of about the same size as the prostate. There is no communication, so far as I can discover, between the duct connecting these structures and the vasa deferentia (PI. XXIII. fig. 5). Genital apparatus of Bergendcdia anomaJa. acc., accessory part of vagina; p. ?, penis-like organ ; pr. ?, prostate-like organ ; s., spiral coils of vagina. For other letters, see p. 318. I can suggest only two explanations of the presence of this second penis-like organ and its accessories. Firstly, that it may be regarded as a prostatic structure which has lost its connection with the penis and developed an intro-mittent terminal part of its own. Such a state of affairs is found in the Cotylean family Diposthiidse ; but in that case both penis and prostate open into a common antrum, and there is what, appears to be a prostate gland in connection with the penis in Bergendcdia, a fact which makes strongly against this view. The second possible explanation is that the organs under discussion are the vestiges of a second penis. The fact that they bear a close resemblance to the functional penis tends to support this view, as does also, I think, a comparison with Cryptocelides of Bergendal [1]. This is the only other Polyclad described, so far as I know, in which two penial organs lie behind each other on the middle line; but, according to Bergendal, they lie behind the female aperture, and, further, both open into a common antrum, whilst in some cases Text-fig. 54. /P r- |