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Show 108 MR. F. F. LAIDLAW ON POLYCLADS [May 26, The dimensions of an adult specimen are as follows:- Length 30 mm. Breadth 17 „ " Mouth," from anterior end ... 14 ,, o* aperture from " mouth" ... 4 ,, $ „ „ male 1 „ This species externally bears a close resemblance to a typical Leptoplana, being perhaps a trifle broader and more leaf-like. Further, its internal anatomy shows it to be distinctly related to the members of that genus, but the presence of two vesicula? seminales with thick muscular walls is sufficient to distinguish it from that somewhat crowded genus. The arrangement of the eye-spots is shown in fig. 3, PI. IX. The pharynx is very large, and extends for a distance of over 8 m m . The gut-branches are numerous and without anastomosis. Genita I Apparatus. Male organs.-The two vasa deferentia each open into the hinder end of the elongated and somewhat convoluted vesicula? seminales (PI. IX. fig. 3, v.s.). These have thick muscular walls consisting of circular fibres, amongst which are found a small number of oval nuclei. The lumen is narrow, and lined with a flattened epithelium. After running forwards for a total distance of about '75 mm., the two vesicula? unite, and their lumen is continued backwards into a median ductus ejaculatorius, which has a length of about •5 m m . For the first part of its course it has thick walls, and its diameter is about equal to that of either of the vesicula?, whilst the epithelium lining its lumen is apparently of a prostatic character. For the last third of its course or thereabouts it becomes much narrower, and has much thinner walls and the epithelium loses its secretory character. Finally, this narrower part of the median ductus, which lies through nearly its whole length at a level dorsal to the vesicula?, opens on to the base of a small penis, which is armed with a downwardly-curved, back-wardly directed stylet. The penis lies at the upper end of a fairly long antrum masculinum. Female apparatus.-The female aperture opens into a wide convoluted vagina, which at first has a course in general in a forward direction, and is provided with rather thick walls, but after a time its walls become thinner and the lumen wider. It then turns backwards, lying dorsal to the first part of its course, and becomes rather narrow, whilst the cells lining it, up to this point ciliated, lose their cilia, and have rather the appearance of secretory cells. W h e n this backwardly-directed part reaches the level of the female aperture, it receives the two uterine ducts through a common opening on its ventral side, and is continued beyond this to end in a small, slightly muscular accessory vesicle. The shell-glands lie close about the aperture. |