Nearly all aspects of the life histories of individual marine-invertebrate species are characterized by ranges of;sizes, seasonal variation, and functional flexibility and phenotypic plasticity in response to varying environmental conditions; that is, they are highly polytypic. Four major areas of polytypy are considered: (1) breeding seasons and cycles vary greatly in time and duration with latitude and from year to year; (2) egg and larval sizes, while showing both genetic and stochastic variability in all species, are often also flexible responses to adult and larval nutrition; (3) modes of development are variable within a number of single species, often reflecting egg-size differences between populations, or even as a result of hatching age of siblings from a single egg mass; and (4) duration of the pelagic larval phase, both before and after the onset of metamorphic competence. Planktotrophic larvae show plastic responses to phytoplankton abundance in their morphologies, and most larvae are flexible in their age at metamorphosis because this complex process requires a more-or-less specialized substratum to induce it for most invertebrate species. New data are presented that provide additional examples of broad flexibility of sibling lecithotrophic larvae to hatch and settle at greatly differing ages. Larvae of the patelloidean gastropod Lottia pelta settled from 8 to 28 days after fertilization, and those of the fissurelloidean Diodora aspera hatched over a three week period from 7 to 30 days post-fertilization; larval settlement had a similarly broad range. Even the planktotrophic-lecithotrophic dichotomy breaks down as increasing numbers of species are found to produce larvae that can metamorphose without feeding (i.e. lecithotrophy), or feed and greatly extend their larval durations (planktotrophy) in the absence of suitable settlement substrata. Invertebrate groups with rigidly canalized life histories are noted to be components of the fouling community, and it is conjectured that an evolutionary history on floating substrata canalized their life histories toward high inbreeding tolerance, often selfing, and brief pelagic larval durations. We note that most life-history theory has considered extremes and major modes in invertebrate development, and suffers from lack of-attention to the abundance of polytypic life-history traits as evolutionary survival mechanisms at the species level. We conclude that response flexibility and plasticity increase both survivorship and fecundity of individuals, while life-history variability increases the likelihood of recruitment across populations and persistence over geological time. More data are needed on the ranges of scalar characters and flexible responses in marine-invertebrate life histories to rigorously evaluate their contributions to evolutionary success.