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Show 1893.] PROSTATE IN THE OLIGOCH.ETA. 481 types is Bothrioneuron (13), where the " Cement-Driise " is attached to the summit of a short diverticulum of the atrium. It seems to me impossible to resist the conviction that we have here a series of stages in the production of a limited " Cement-Bruse out of a continuous glandular investment of the atrium. In Moniligaster, whose atrium as regards the points under discussion is so like that of Branchiura, the glaudular cells which invest it externally perforate the muscular layer to reach the inner epithelium ; I imagine that this will be found to be the case with Branchiura too. A reduction of this investing layer to a restricted region of the atrium brings about the Cement-Driise of Tubifex; a greater separation between the masses of glandular cells would produce the state of affairs characteristic of Telmatodrilus. It is noticeable that the absence of peritoneum upon the atrium of Branchiura is seen in Tubifex in the region of tbe Cement-Driise ; why this should be I cannot suggest; but that it is so is a further reason for the justice of m y comparisons. In the family Lumbriculidae there are indications of a similar series of stages in the conversion of a complete glandular covering of the atrium into a more partial one ; but the stages are fewrer than in the Tubificidae, in fact there are only, so far as our present knowledge of the group goes, two such stages. The more usual condition is seen in the majority; in Rhynchelmis, for example, the atrium is lined by a layer of cells, outside which are masses of pear-shaped cells which are aggregated to some extent into separate masses; the after all rather slight indications of discontinuity between the masses of cells which clothe the atrium in Rhynchelmis are still further emphasized in Sutroa. In this Annelid (8) the atrium is invested by very distinctly separate masses of glandular pear-shaped cells ; the entire atrium is covered by a thin muscular layer which might be regarded as the peritoneal investment ; between this supposed peritoneum and the glandular coat of the atrium lie masses of developing sperm ; the structure therefore, whatever be its morphological nature, is, in function, a sperm-sac. It seems to m e that the two families Tubificidae and Lumbriculidae are nearly allied, inany case no one has disputed the morphological identity of the organs termed atria in both; it is therefore reasonable, at least pending further information, to assume that the atria correspond in detail, and in this case the Lumbriculidae, like the Tubificid genus Branchiura, have an atrium which is frequently devoid of peritoneum. The intermediate condition between the atrium in the Lumbriculidae and that in the higher Oligochaeta is afforded by the Moniligastridae. Moniligaster itself has an atrium which seems to be identical in all essentials with that of Branchiura and the Lumbriculidae ; it has two layers of cells which are separated by a well-developed muscular layer. I have shown (9) that the outer layer of cells sends processes through the muscular layer, which therefore are in a position to void their contents into the lumen of the atrium. I myself have only seen this arrangement |