Larvae of the aeolid nudibranch Phestilla sibogae metamorphose in response to a water-soluble factor derived from their adult prey, the coral Porites compressa. Destruction of the velum, a ciliated larval swimming and feeding organ, is one of the major morphogenic events that occurs during metamorphosis. Previous work established that whole larvae metamorphose in response to the natural coral-derived inducer (CI) and to choline, and shed the velum without completing metamorphosis in response to hydrogen peroxide (H2O2). In the present study, excised velar lobes were maintained in vitro while their responses to known inducers of metamorphosis were examined. In this way direct tissue-level morphogenic effects of inducers could be discerned. Isolated velar lobes were unaffected by CI and by choline (5 mM). “Donor” larvae (each of which had supplied a single velar lobe used in these experiments) and unoperated controls lost their vela in response to both treatments. These results are consistent with the hypothesis that morphogenic effects of CI are mediated by the central nervous system. H2O2 (0.2 mM) caused disintegration of isolated velar lobes, and caused nearly all donor and unoperated larvae to shed their vela. © 1993 Wiley-Liss, Inc.