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Show 1903.] POLYCLADS OF THE " SKEAT EXPEDITION." 307 tlie point where the uteri open into it, the muscular walls, which are elsewhere feebly developed along its course, become thicker, but not very markedly so (cf. L. californica von Plehn). The two uteri (at.) unite as they enter it from below, and behind them there is a very small accessory vesicle (acc.ves.) witli muscular walls. The genus Leptoplana, as a.t present constituted, contains a considerable number of species, many of them only referred by Lang with doubt to the genus. In the majority of the species, the anatomy of which has been investigated by means of serial sections, the genital apparatus presents the following characters :- The penis is directed backwards. The vasa deferentia open into a muscular vesicula seminalis ; from this the ductus ejaculatorius runs backwards to open into a prostate organ, which maybe chambered; leaving this, the duct runs back into the penis, which may be armed with a cliitinous stylet. The vagina runs back from the antrum femininum, receives the secretion of the shell-glands, and further back the two uteri open into it ventrally, usually by a common duct. An accessory vesicle is generally present. The most familiar species of the genus, L. tremellaris, differs from the majority of species in the structure of its male apparatus sufficiently to permit us to put it on one side to form of itself a section of the genus further characterised by the possession of a ventral sucker between the genital openings. Another species, L. subviriclis von Plehn [8] ( - L . pardalis milii [4]), approaches Discocelis tigrina in the structure of its female organs, and accordingly we may put this species also in a section apart. The remaining species I have attempted to classify below, so far as is at present possible, according to the structure of the penis and prostate. I trust that I will not be thought to have laid too much stress on the structure of these organs in dealing with this or other genera. It seems to me that, of all the characters that present themselves for classifying this order of Turbellaria, these are the most useful, and that they are as reliable as any of the other characters employed for this purpose, such as structure of the pharynx, number and arrangement of eye-spots, presence or absence of tentacles, &c. &c. I propose, then, to group the species in two sections, A, and B, so far as our knowledge of their anatomy permits:- A. Penis provided with a stylet. (a) Prostate complicated by radially arranged " Drusen-kanale." 1. L. kukenthali v. Plehn. Spitzbergen [7]. (b) Prostate divided into chambers lying parallel to the ductus ejaculatorius. 2. L. vitrea Lang. Mediterranean 16 3. L. alcinoi Schmidt. „ [6 |