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Show 316 MR. F. F. LAIDLAW ON THE [Apr. 21, a loss to explain satisfactorily. In the first place, the testes and ovaries are very immature, and apparently only the terminal parts of the genital ducts are developed. On the other hand, there is, immediately over the penis, a large mass of mature spermatozoa lying in a chamber the character of which cannot be determined from my specimens, since it appears in places to have indications of a proper lining-epithelium of its own, and again in places seems to be merely a gap in the parenchymatous tissue. There are very faint indications of a duct running from this chamber down in the direction of the penis, but this duct cannot be traced far. Woodworth [18] suggests in connection with the specimens of Diposthus described by him, that they were in a late stage of sexual activity, and that in consequence of this the sexual organs were in a reduced condition. Possibly in my specimens of Asthenoceros a similar state of affairs occurs, but, as I have already stated, testes and ovaries in a very immature state are present. There is also a possibility that the ripe spermatozoa may be derived from another individual by hypodermic injection ; but I do not think this is the case-firstly, because the penis is not armed with a stylet; and, secondly, because the spermatozoa lie over the penis, and because, as already stated, there are traces of a duct running towards the penis from the chamber in which they lie. The terminal parts of the male apparatus histologically resemble those of Diposthus. The prostate and penis are separated into two distinct organs, " both of which are doubtless intromittent," both opening by a single gonopore. The prostate lies immediately in front, the penis directly over the aperture. It is rather feebly supplied with muscle-fibres, and the secretory cells lie in the middle of the organ, but there is no lumen apparent (cf. Diposthus corcillicola). The penis is much more muscular, and on the outer side has a very definite series of circular and longitudinal fibres. Nuclei in it too are much more numerous (PI. XXIII. fig. 7). In connection with the female organs there are traces of two pairs of uterine vesicles. The uteri are very small and difficult to distinguish from the surrounding tissue, and the ducts running to them are merely solid rods of cells of an embryonic appearance. The uteri open into the hinder end of the vagina. The antrum is deep, and widens at its upper end where it receives the secretion of the shell-glands. The body epithelium is very densely crowded with small rhabdites, and on the dorsal side especially with pseudorha bdites. Scattered through the parenchyma, more particularly in the region of the sucker, are numbers of small rounded darkly-staining cells the nature of which is doubtful. The ventral surface of the body projects immediately behind the pharynx into a prominent median ridge which carries the gonopores and the sucker; behind the sucker the ridge disappears. I have given as complete an account as possible of the cha- |