Treatise Online no. 61: Part N, Revised, Volume 1, Chapter 4: The early shell: Ontogeny, features, and evolution

that the embryonic, larval, metamorphic, and postlarval (juvenile to adult) phases represent major developmental alternations in this sense.The term phase is herein restricted to refer to a certain developmental period.Comparisons between developmental trajectories of organisms require reference points marking the onset and offset of phases.Traditionally, these boundaries have been defined on the basis of morphologically recognizable markers, such as the appearance and disappearance of organs or, in the case of molluscan shell development, changes in structure, sculpture, or direction of growth.The term stage is herein restricted to refer to these morphologically recognizable units of the shell.Defining clear reference points for ontogenies is important because onsets, offsets, and development rates can vary among organs of a species as well as among homologous organs in distinct species (heterochrony).This may lead to conflicting concepts of developmental phases.For example, Gros, FreNkiel, and Mouëza (1997) observed differences in the early development of gills, siphonal structures, and shells of various heterodont bivalves with similar reproductive modes.Their observations led them to distinguish two metamorphic phases, late larval and early postlarval, and to raise the question of which criterion should be used to mark the offset of the larval phase.Shell development is similarly diverse, including discrete shell stages within both larval and postlarval phases, dislodgements of shell features between one and the other phase, or continuity of development beyond metamorphosis.