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SUMMARY.

A description is given of the anatomy and histology of the genital system of the terrestrial prosobranch Pomatias elegans, of the structure of its egg capsule and of the development of the embryo until the time of hatching. Reproduction and development are compared with those processes in marine forms.

The breeding season is prolonged, extending from February until August, and, correlated with this, there is little seasonal variation in the size of the genital system.

The male is more specialized than in the related Littorina spp. because the pallial and penial ducts are completely closed, but in the female specialization has led to a secondary opening of the pallial duct to assist the expulsion of the large egg capsule. This has necessitated changes of structure in the organ concerned with copulation and the storage of sperm. The sac homologous with the receptaculum in other species has developed a direct opening into the mantle cavity and acts as a posteriorly situated bursa copulatrix and the gonadial duct is comparatively wide and functions as the receptaculum seminis. There is no gonopericardial duct.

The extremely glandular pallial oviduct secretes a capsule 2 mm. in diameter around the single small egg (0±13 mm. diameter) and it is the albumen and the gelatinous capsule wall that are used for food during the three months of intra-capsular development and not a large accumulation of yolk. Absorption of water and salts from the surrounding soil also occurs and there is some factor present in natural soil water which is necessary for development. Correlated with the lack of yolk, invagination and epiboly are of equal importance in gastrulation. Early in this process the mesoderm mother cell, 4D, moves between the ectoderm and endoderm.

Intra-capsular development continues until the animal is a miniature adult, and the trochophore and veliger stages are considerably modified. No spacial larval sensory, excretory or respiratory organs are formed, but a large embryonic structure, the cephalic mass, develops in the velar region. This has an absorptive function. The cephalic mass begins to differentiate during the early veliger stage, and is most fully developed during the mid and late veliger stages; it is steadily reduced and has disappeared completely by the time of hatching. As the cephalic mass becomes smaller relative to the size of the animal, the digestive gland, which develops precociously, becomes larger.

Development of the principal organs is as in prosobranchs like Haliotis and Patella. The mantle cavity develops in the early and mid veliger stages partly from the pretorsional right member of a pair of ectodermal invaginations which are formed in the postero-ventral part of the visceral mass, one on each side of the rectum, and partly as a result of the growth of the mantle skirt. The kidney and pericardium develop from adjacent mesodermal rudiments on the pretorsional right side of the embryo and the fact that these structures were originally paired is shown by the presence of a vestige on the left. The principal ganglia of the nervous system are all formed separately by delamination from the ectoderm.

The widest departure from the course of development described in other prosobranchs is found in the process of torsion. It is the result of differential growth alone and at the time it occurs, no muscles capable of contraction are differentiated.