Larvae from diverse marine-invertebrate phyla are able to respond rapidly to environmental cues to settlement and to undergo very rapid metamorphic morphogenesis because they share the developmental trait of metamorphic competence. The competent state, characteristic of larvae as diverse as those of cnidarian planulae, molluscan veligers, and barnacle cyprids, is one in which nearly all requisite juvenile characters are present in the larva prior to settlement. Thus metamorphosis, in response to more or less specific environmental cues (inducers), is mainly restricted to loss of larva-specific structures and physiological processes. Competent larvae of two “model marine invertebrates” studied in the authors' laboratory, the serpulid polychaete Hydroides elegans and the nudibranch Phestilla sibogae, complete metamorphosis in about 12 and 20 hr, respectively. Furthermore, little or no de novo gene action appears to be required during the metamorphic induction process in these species. Contrasting greatly with the slow, hormonally regulated metamorphic transitions of vertebrates and insects, competence and consequent rapid metamorphosis in marine invertebrate larvae are conjectured to have arisen in diverse phylogenetic clades because they allow larvae to continue to swim and feed in the planktonic realm while simultaneously permitting extremely fast morphological transition from larval locomotory and feeding modes to a different set of such modes that are adaptive to life on the sea bottom.