1. The atrium in two species of the Naididae is described. The prostate in Nais elinguis forms the wall of the vas deferens; in Slavina punjabensis it covers the atrium. The atrium is formed as an ectodermal invagination and always occupies a position at the end of a short vas deferens. The atrial epitbehium in the sexually mature Stylaria lacustris and Slavina punjabensis is replaced by the prostatic secretion which passes through it ; it is not really composed of glandular cells as has been so far considered.
2. The prostate cells in the Naididae may surround the atrium or the vas deferens or both. The cells appear to penetrate into the muscle-fibres of the atrial wall in order to directly communicate with the atrial epithelium. They also become more or less completely changed in the late stage of sexual maturity into the secretion and lose their identity. The prostate cells are peritoneal in origin. The prostate may be absent or rudimentary in some species of the Naididae.
3. Various stages in the development of the sperm-duct in Tubifex barbatus are described. The atrium is shown to be a part of the vas deferens and hence mesodermal in origin. The prostate lies near the seminal funnel on the ventral side of the vas deferens just above the ovary ; in the early condition it is a deeply stained mass of nuclei with a little cytoplasm and continuous with the peritoneal layer of the vas deferens in front and behind.
4. The atrium and the prostate in Tubifex tubifex and Limnodrilus udekemianus are described. The prostate is peculiar in the species of the Tubificidae iuvestigated in the fact that the secretion of a fibrillar appearance forms the centre of the gland, while the cells occupy the periphery. The fibrillar secretion flows into the atrial epithelium, through a regular passage or opening where the muscular wall is absent. This appearance probably led previous workers to suppose that the prostate cells open by long ductules at the point of communication with the atrium. There is no evidence whatsoever that the prostate arises as an outgrowth from the atrial epithelium ; on the other hand it is obviously peritoneal in origin.
5. The cytoplasm in the prostate cells is basophil as distinguished from the acidophil central part with the secretion, and in Tubifex tubifex during the advanced condition of the gland generally contains basophilous granules, which pass with the fibrillar secretion as secretory products into the atrium. The prostate cells in the advanced sexual condition lose their structure and do not show mitochondria.
6. The atrial epithehium in all the Species of the Tubificidae considered is not composed of gland-cells. It is on account of the prostatic secretion, which passes through it into the atrial lumen, that it degenerates and its cells break down.
7. The prostatic secretion in the Naididae and Tubificidae appears as a fluid of thick consistency. Its function is mainly to cement or unite sperms received in the atrium. It does not contain mucin, but in sections of Tubifex tubifex prepared according to the Mann-Kopsch method the central conducting part of the gland and a portion of the atrial wall show the presence of a large amount of fat, mainly olein, or some lipoid substance, which reduces osmium tetroxide, and thus becomes deep black like the dictyosumes of the Golgi apparatus.
8. The homologies of the atrium and the prostate are briefly discussed in the various families of the Microdrili.
9. Peculiar features in the structure of the vas deferens in Tubifex tubifex and Limnodrilus udekemianus are described.