Ultrastructural descriptions of the oviduct, fertilization canal, seminal receptacle, ovovitelline duct, vitelline reservoir, ootype, Mehlis' gland, proximal uterus, and neurosecretory elements associated with egg-forming ducts are given for the progenetic spathebothriidean tapeworm, Diplocotyle olrikii from the body cavity of Gammarus oceanicus. The functional significance of cortical granules of the oocyte, as necessary elements for joining vitelline material to an oocyte in the ovovitelline duct, is established. The proximal ootype has a vesicular epithelium and is the site of initial, nascent eggshell formation. Precursors of nascent eggshell are vesicles, synthesized in both the proximal ootype wall and vitelline cytoplasm that become associated with the newly formed shell. Major shell structure comes from subsequent deposition of shell globules from a disintegration of vitelline clusters. Mehlis' gland has a single secretory cell type. Secretory granules from Mehlis' gland become associated with the developing egg that passes through to the distal ootype and proximal uterus where egg-formation is completed. It is not known, however, whether Mehlis' gland secretion promotes breakdown of free vitelline cells, liberation of shell globules, confluence of shell globules on the developing eggshell or provides further structural components for the shell. Despite some differences in ootype morphology, the basic process of eggshell formation in D. olrikii may share much in common with the Pseudophyllidea and Caryophyllidea. Small vesicles and dense-core vesicles are in nerve terminals near duct musculature. Nerve terminals with large dense vesicles are described near, in, and within the seminal receptacle, fertilization canal and distal ootype. The possible physiological effects of exocrine neurosecretions are discussed.
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