Thompson (1967) defines three developmental types for Opisthobranchia with representative nudibranchs in each category. The types are: 1.) planktotrophic larvae which are obligatory plankton feeders prior to progressive metamorphosis, 2.) lecithotrophic larvae which may feed on the plankton, but do not need to do so in order to metamorphose, 3.) direct development which results in hatching of a post veliger, benthic juvenile. Tardy (1970) feels that Thompson's Type 3 development is artificial. Thus, he incorporates direct development into the lecithotrophic developmental type. Tardy then proposes a classification of metamorphic types based on larval shell type (Thompson, 1961) and on larval feeding behavior (ie. lecithotrophic vs. planktotrophic larvae). I agree with Thompson's (1967) distinction between direct and lecithotrophic developmental types in the sense that they represent ecologically diverse ontogenies. The major explicit assumption in the above definition of planktotrophic larvae is that feeding is a necessary prerequisite to progressive metamorphosis. However, both Thompson (1967) and Tardy (1970) appropriately note that metamorphic observations of planktotrophic larvae are fragmentary and circumstantial. Thus, the definition of planktotrophic larvae rests its credibility on larval morphology at hatching, and the implicit assumption that energy is required (ie. feeding) to develop the organs necessary to accommodate the functional transition to the adult mode of life. The present study describes the early embryology, larval development, veliger morphology and feeding behavior of Hermissenda crassicornis and Aeolidia papillosa. Veligers of the facelinid, Hermissenda crassicornis and the aeolid, Aeolidia papillosa have striated, Type 1 (coiled) larval shells and fall within Thompson's (1967) definition of planktotrophic larvae. Veligers of both species remain planktonic for two to five days after hatching. They subsequently become epibenthic swimmers and discard their larval shells. There is considerable variation in the amount of yolk reserves in the gut and diverticulae of recently hatched veligers. An individual egg mass yields larvae with and without yolk reserves. The results show that shell length frequency of hatching larvae is distributed bimodally. There is a larval dimorphism based on shell length at hatching, the presence of yolk reserves and feeding ability. Feeding larvae are found to differ with respect to diet; the difference being associated with shell length of the larvae in relation to food particle size. The results are discussed in a comparative review of larval development in the Eolidoidea. Secondly, the relative dependency of nudibranch larvae on feeding ability is discussed with respect to the morphological and developmental categorization of opisthobranch metamorphic responses. The functional and ecological considerations of feeding in gastropod and selected invertebrate larvae are discussed with respect to the evolution of larval strategy and life cycle phenomena.