Vance (1973a, b) argued that among the possible range of developmental strategies available to marine invertebrates, only the extremes of obligate planktotrophy and obligate lecithotrophy are evolutionarily stable. Vance's model, relating reproductive 'efficiency' to egg size (in terms of energetic content), predation rate, and prefeeding (lecithotrophic) vs feeding (planktotrophic) larval periods, has been a source of much discussion and debate since its inception (e.g. Underwood, 1974; Vance, 1974; Christiansen & Fenchel, 1979; Obrebski, 1979; Williams, 1980; Jablonski & Lutz, 1983; Strathmann, 1978, 1985; Todd, 1985). Subsequent publications have continued to dwell mainly on potential selective factors and the extremes of larval developmental type (i.e. obligate planktotrophy or obligate non-pelagic lecithotrophy). For the most part, these investigations have ignored questions concerning how a transition from one larval type to another would be accomplished in morphological and functional terms. Nonetheless, the consensus persists that small eggs and planktotrophy are the primitive (or ancestral) condition, and that lecithotrophy is the more advanced evolutionary derivative (see Strathmann, 1978,1985).