The male copulatory apparatus in an opisthobranch mollusc, Runcina

The copulatory apparatus of a primitive opisthobranch, Runcina, is described. The apparatus is comprised of the following organs: the spermatic bulb, the prostrate, the penis and the penial sac. The spermatic bulb wall consists of cuboidal epithelium with forked microvilli and densely arranged cilia. Prior to copulation the interior is tightly packed with sperm. The prostate is lined with alternating glandular and supporting cells, the latter being compressed but with a mushroom-shaped apex bearing a few forked microvilli and many cilia. The glandular cells produce differing secretions, each cell producing a single type. Large paracrystalline structures enclosed in cells close to the penial area are particularly striking. Considerable amounts of the secretory products are accumulated in the protruding cell apices. One type of inclusion is found at a later stage, packed around the sperm mass within the spermatophore; its function, and the fate of the other secretions is not yet clear. The epithelium of the penis is of more columnar structure, covered with forked microvilli and extremely long cilia which are anchored by long rootlets in the cells. Some of the cells contain large electron-dense secretory granules, others hold accumulations of small secretory vesicles in their apices. It seems likely that these contribute towards the outer layer of spermatophore. The wall of the penial sac is lined by one to two rows of flat-cuboidal cells bearing sparse forked microvilli and cilia.