Typescript. Includes vita and abstract. Bibliography: Includes bibliographical references. Content: Patterns of development in nudibranch mollusks from the northeast Pacific Ocean, with regional comparisons. -- Lecithotrophic development in Doto amyra (Nudibranchia: dendronotacea), with a review of development in the genus. -- Unusually large polar bodies in an aeolid nudibranch, a novel mechanism for producing extra-embryonic yolk reserves. xiv, 237 leaves : ill. ; 29 cm. / Biogeographic patterns of developmental mode in marine invertebrates have been examined with respect to latitude, depth, and general habitat type. Regional comparisons, which might reveal the influence of specific ecological mechanisms on mode of development, are few. The present study was undertaken to: 1) characterize early development, especially its mode, in nudibranch molluscs from the cold temperate waters of the northeast Pacific Ocean: 2) compare the development of these species to that of nudibranchs from other geographic regions: and 3) attempt to explain the observed patterns on the basis of regional differences in hydrography, geology, and primary production. Observations of egg size, embryonic development and hatching larvae were made for 30 species and were supplemented with data from the literature. All data for other regions were obtained from the literature. Developmental mode was determined for 69 NE Pacific species, over half the known fauna. sixty-seven (97%) have planktotrophic development and two (Doto amyra and the introduced Tenellia adspersa) produce short-term lecithotrophic larvae. No evidence for nurse-eggs was obtained in this study, and only the egg capsules of the aeolid Cuthona lagunae were found to contain extra-zygotic yolk reserves. These took the novel form of unusually large, yolk-filled polar bodies that were ingested by the embryonic veligers. Planktotrophic species with egg-shaped, inflated larval shells develop from larger eggs than those of species with typical spiral shells. The former, however, have shorter embryonic periods, owing, in part, to their smaller egg masses; many also hatch with eyespots. World-wide I conclude that planktotrophy, at least among nudibranchs, is most common in regions with relatively slow currents, high standing stocks of phytoplankton, and large expanses of habitat suitable for the adults. This is documented by evidence from the NE Pacific, NE u.s., Britain, NW Red Sea, and NW Mediterranean. Nonfeeding modes of development tend to predominate in areas with fast boundary currents, oligotrophic waters, rapid changes in physical aspects of the adult environment, or in areas formerly in contact with polar waters (e.g., SE U.S., Marshall Is., New South Wales). Latitude and its correlates do not sufficiently account for the observed regional differences in mode of development.