Tritiated thymidine autoradiography was used to distinguish exogenous from endogenous sperm and to determine the translocations of labeled sperm within the simultaneously hermaphroditic, reciprocally copulating gastropod Phyllaplysia taylori. Heavily labeled sperm start leaving the ovotestis within 14 days after injection of H3‐thymidine. These endogenous sperm are stored, evidently less than 20 days, in the ampulla prior to copulation. During copulation the endogenous sperm are tightly bound to one another by extracellular material forming a seminal strand. This seminal strand is moved by ciliary action along the spermoviduct fold and genital groove to the tip of the extended penis. As the seminal strand issues from the tip of the labeled animal's penis into the copulatory duct of an unlabeled mate, the extracellular material of the strand dissolves, releasing the now exogenous sperm (some of which are labeled). Most of the labeled exogenous sperm are found in the seminal receptacle within two hours after copulation. Within five hours some of these sperm have become oriented with their heads buried into the receptacle lining. Surplus exogenous sperm flow into the copulatory bursa where they are destroyed.

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The wall of the gametolytic gland in Aplysia and Phyllaplysia consists of a muscular layer lined with an epithelium incorporating four distinct cell types, some of which show intense secretory activity. The cytoplasm of these cells, rich in RNA, has a peripheral layer of densely-packed mitochondria in association with a smooth endoplasmic reticulum. Lipid globules, perhaps resulting from absorption of lumen contents by microvilli, appear in the cytoplasm in the course of growth and reproductive activity. A basal band of fibrillar material appears to serve as a cytoplasmic skeleton. The elaboration of cell types, extensive areas of cytoplasmic RNA, and vigorous secretory activity suggest important roles for this organ such as development of food reserves, recycling of sexual materials of high physiological value, and interaction with the bag cells of the nervous system in coordinating ovulation with the availability of capacitated, exogenous sperm.

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