The Structure and Mode of Functioning of the Reproductive Organs of Tritonia hombergi (Gastropoda Opisthobranchia)

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ABSTRACT Routine observations on Tritonia hombergi Cuvier dredged from an area off the south coast of the Isle of Man indicate that the species has an annual life-cycle similar to that of many dorid nudibranchs. The ovotestis communicates with the anterior genital mass through numbers of collecting tubules which unite to form a stout hermaphrodite duct. This duct functions as a vesicula seminalis in adult specimens; spermatozoa stored here are physiologically immature. The hermaphrodite duct bifurcates to form male and female ducts; the region of the bifurcation forms a valve ensuring that oocytes shall not enter the purely male regions of the genital system during oviposition. The male duct is the glandular and muscular vas deferens which connects with the base of the protrusible penis. The oviduct opens into the lumen of the mucus-gland. A region of convoluted tubules close to the point of the opening of the oviduct into the mucus-gland is the albumen-gland. The bursa copulatrix is a blind tubular invagination of the lateral body-wall which in mature specimens stores spermatozoa received at copulation. There are thus three external genital openings. Copulation is almost invariably reciprocal, the penis of each individual being inserted into the bursa copulatrix of the other. Secretions from the glandular linings of the bursa and of the vas deferens render spermatozoa exchanged at copulation physiologically mature; they are active and capable of fertilizing oocytes. At oviposition oocytes are expelled from the ovotestis and transported by cilia to the main hermaphrodite duct. They then travel (through the dense masses of spermatozoa in this duct) along a narrow longitudinal ciliated tract of lining epithelium; this phenomenon was clearly observed in vivisected individuals. At the hermaphrodite valve the oocytes are deflected into the oviduct and are carried by cilia into the mucusgland. In the lumen of this organ they meet active ‘foreign’ spermatozoa which have made their way from the bursa copulatrix. The oocytes and spermatozoa are now conducted along a tortuous predetermined ciliary path through the albumen-gland (where the primary egg-cases are secreted and where fertilization occurs), and then through the peripheral tubules of the mucus-gland (where successive mucous coats are added). The completed egg-string is expelled through the external female aperture and forms a ribbon which is attached to the substratum by pressure of the parent’s foot upon it. Active spermatozoa may be observed moving within the primary egg-cases for some days after oviposition.