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Show 1893.] PROSTATE IN THE OLIGOCH.ETA. 483 atria of Besmogaster as offering an intermediate stage between Moniligaster and the higher Oligochaeta. In the Oligochaeta we have an atrium consisting of two layers of epithelium, covered either by a simple thin peritoneum only, or (in the Eudrilidae) consisting of thick muscular layers themselves covered with the peritoneal epithelium. In a few simplified genera of Megascolicidae the lining of the atrium is reduced to the inner layer of epithelium only : these are Ocnerodrilus, Gordiodrilus, Pggmceodri-lus, and Kerria (an Acanthodrilid). It is, 1 imagine, fairly probable that these forms are really, so far as concerns the atrium, degenerate in character; they show no real resemblance to the " Limicolae " as I was at first inclined to believe; for as I now show that the same constituents are present in the atrium of the lower as of the higher Oligochaeta, the single layer of cells is a difference from both. At one time the family Geoscolicidae was defined by having no atria like other families such as the Megascolicidae ; many genera, however, do possess organs connected with the external pores of the sperm-ducts which seem to be referable to this category. Eosa, however, in an account of the minute anatomy of these organs in Kynotus (15), is disposed to regard them as morphologically different from the atria of the Megascolicidae. Unfortunately the description of the structure of the organs in question in Kynotus is unaccompanied by any illustrations, so that it is a little difficult to follow. The terminal male apparatus consists of a muscular bulb (" bursa propulsatoria ") attached to the walls of the segment, on to which it opens by a retractor muscle ; from this muscular sac arises a glandular tube of some length ; the muscular sac is divided into two chambers by a transverse septum; from the dorsal oue of these is given off the glandular appendix; the appendix is at first lined by a single layer of cells, and it is into this part of it that the sperm-duct opens ; further back the lining epithelium is composed of two layers, the outer one being a mass of pyriform glandular cells. Rosa is of opinion that this structure is not the homologue of the atria of the Megascolicidae ; its resemblances, he thinks, are superficial and, such as they are, are due to convergence. The main reasons which Rosa gives for this opinion are three :- (1) The fact that these " pseudoprostates " are found only in a much specialized group of the Geoscolicidae; (2) their difference in structure from the true " prostates " of the Megascolicidae ; (3) the great resemblance which they show to the sacs accompanying the copulatory setae of the same worms. The second reason is further expanded into the actual points of difference ; these are the following :-the presence of a retractor, the complicated structure of the terminal sac, the mode of connection of the sperm-ducts with the glandular appendix, and the disposition of the glandular part. It appears to m e that the differences are not so great as Rosa would have us believe; it is true that if we compare, as he suggests, Pontodrilus with Kynotus, the differences are more striking than the resemblances. I, however, decline to regard the |