Comparative morphologic and biomineralogical studies on the relationship between mantle differentiation and early shell formation in Mollusca

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In this study, we have examined the relationship between mantle differentiation and shell formation in early developmental satges of Recent and fossil mollusks from the viewpoints of comparative morphology and biomineralogy, and the results of observations were considered for their morphogenetic, larval ecological and phylogenetic implications. The results are summarized as below.A. Embryology and larval shell development of Recent Gastropoda. SEM studies of a freshwater snail Lymnaea stagnalis in the early stages reveal that the early larval shell was not formed accretionarily, and that the biomineralization process in the early stage differs from that in the post-metamorphosis stage.B. Larval shell formation in Recent Bivalvia SEM and TEM studies of a pholadid bivalve Zirfaea subconstricta in the trochophore and early veliger larval stages indicate that outer surface of secreting cells contribute a shell pellicle and functions as an adhering site of tendon cells at the stage of non-biomineralization.C. Embryonic shell development of Mesozoic Ammonoidea Exceptionally well-preserved embryonic shell fossils of Mesozoic ammonites were examined for their shell wall structure and ornament by means of SEM. The results demonstrate the succession of early shell wall development starting with a horny primary shell and ending in the secretion of a nacreous swelling at the aperture via secretion of inner prismatic and outer prismatic layers. Irregular distribution of microtubercles on the outer prismatic layer suggest that they were formed by the aid of a shell gland but by non-biological precipitation by egg-fluid.