III. Reproductive System
Serpulorbis, like the rest of the vermetid genera, is incubatory. The eggs are retained in capsules, attached to the inner surface of the shell, along the median pallial slit of the female. From 3 to 5 capsules are generally produced at one time, each containing about 10 eggs, 0.25 mm in diameter. The free-swimming stage is entirely lost, and the velum of the embryo much reduced in size. On emerging from the capsule membrane, the embryo creeps out of the aperture of the female shell, and is able for a limited time to crawl about, before attaching itself to the substratum and developing the uncoiled adult shell. The male is aphallic, and the sperms are shed from the vas deferens directly into the mantle cavity and carried out by the exhalant current. The female is thus fertilized by the entry of current-borne sperms with the inhalant pallial current. The pallial genital duct is for most of its length widely open for the entry of sperms from the pallial cavity. A similar case of current fertilization with an unclosed genital duct is described by Fretter (1946) in Turritella in which the capsule gland and the albumen gland are both widely open along the ventral surface. The pallial genital duct primitively originated as an unclosed groove, and examples of incomplete closure with current fertilization are to be found also in the Cerithiidae (Zeacumantus, unpublished observation) associated with well-protected pallial apertures in silty habitats, as well as in the sessile vermetids. These cases may have arisen by the re-acquisition of a primitive condition; in the Strombacea on the other hand (Morton, 1949b) copulation occurs, and the unclosed capsule gland is to be attributed to a primitive survival.
The female genital ducts of Serpulorbis zelandicus are illustrated in Figs. 10–12. The narrow ovarian duct (50μ) lined with ciliated epithelium is capable of great distention with the passage of eggs. It opens through the right wall of the albumen gland, which forms the first portion of the pallial oviduct. The lumen of the albumen gland (Fig. 11, ALB.G.) is curved and slit-shaped, closed along the lower edge, and lined by a single row of tall, columnar cells (30μ) filled with secretion spherules staining light-brown with Van Giesen's. These cells secrete the albumen coat surrounding the eggs after fertilization. The wall of the albumen gland is strongly ciliated, and sperms are carried backward from the capsule gland along the ventral side of the lumen to the receptaculum. This is represented by a small spherical pouch, 1.5mm.–2mm. in diameter, attached dorsally to the posterior end of the albumen gland. The lining is of short, cubical epithelium, and a dense row of haematoxylin-stained sperm heads is generally attached to the surface of the cells. It may be suspected that a chemotactic stimulus is responsible for the invariable aggregation of current-borne sperms in this part of the genital duct.
The capsule gland (Figs. 10, 12, CPS.) is much larger than the albumen gland. It forms a pouch-shaped organ, 9–10 mm. long, pale yellow in colour, open along its whole ventral surface into the pallial cavity. The epithelial cells are of two sorts, narrow ciliated cells, with a short ciliary coat (50μ) and dark rod-shaped nuclei, and columnar gland cells, 50μ tall, whose secretion stains lightly with haematoxylin. A single egg capsule occupies the whole lumen of the gland, and the
group of eggs, with its secreted envelope, is then attached directly to the shell, the first formed capsule most anteriorly. As the capsule is attached, the capsule gland cilia rotate it to twist off a short stalk, and albumen coat is also twisted into a chalaziform strand. The embryos move about to some degree within the capsules, and the membrane is finally ruptured by the sharp edge of the embryonic operculum, which has attained its maximum size at the time of liberation.