1. The genital organs of Stylaria lacustris are described in detail. The vas deferens opens into the atrium on the anterior face near the opening of the ejaculatory duct and not at the top as described by all the previous authors. The prostate surrounds not only the atrium but also the vas deferens in segment 6.

2. The prostate secretion passes through the atrial epithelium, which consequently hypertrophies and disappears

3. The development of the genital organs proceeds with great rapidity when the sexual phase appears, which occurs only once a, year from the end of September to the beginning of December. There is no long intervening period between the development of the gonads, and other genital organs.

4. The order of development seems to be connected with the time or order of their functioning.

5. The gonads are peritoneal in origin. The sperm-sac and orisac are large portions of the body-cavity enclosed by the extension backwards of septa ⅚ and 6/7 respectively.

The yolk-mass is formed by a process of metabolic change in the cytoplasm of some of the ova.

6. The sperm-duct is partly peritoneal in origin and partly an ectodermal invagination. The funnel and the vas deferens rudiments arise by a proliferation of the peritoneal cells on the anterior face of septum ⅚, which assumes the form of a deeply shining plate of columnar cells with prominent nuclei. This after the funnel rudiment becomes the sperm-cord and penetrates the septum in front of the ovary, reaching near the body-wall the atrial rudiment, which is soon formed as an ec todermal invagination.

The prostate cells arise from the peritoneum near the rentral body-wal1 of the sixth segment in the neighbourhood of the atrial rudiment.

7. The rudimentary female funnel, which opens ont at the female opening, arises as, an outgrowth from the peritoneum at the base of septum 6/7.

8. The spermatheca srises as an invagination from the ectoderm. I agree with Bergh that the sperm, zthecae are to be considered as new structures, and not phylogenetically connected with the genital ducts as Gatenby supposes to be the case in Tubifex rivulorum.

9. A fern stages obtained showing the development of these organs in Nais e1inguis confirm the above observntions.

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