My research tested hypotheses about the evolutionary origin of a novel feature by modification of development. The novelty is the growing larval shell of gastropod molluscs, which emerged when gastropod larvae acquired the ability to feed. One hypothesis states that the growing larval shell in the Heterobranchia is a continuation of the embryonic phase of shell secretion. The second hypothesis states that the larval shell in the Caenogastropoda may be a precocious juvenile shell. These hypotheses implicate heterochrony. To test these hypotheses, I examined ultrastructural features of the shell-secreting cells of two or three life history stages in a member of each of four clades of gastropods: the Patellogastropoda, Vetigastropoda, Caenogastropoda, and Heterobranchia. My results are consistent with the first hypothesis, but I found no ultrastructural support for the second hypothesis. These results provide the most comprehensive comparative data set on the ontogeny of shell-secreting cells for the Gastropoda. / Graduate / 0433

Predatory gastropods (Neogastropoda) feed with a proboscis (elongate snout) and complex foregut. The presence of developmental modules (semi-autonomous components) in foregut development may have facilitated emergence of predatory feeding. In species with indirect development (feeding larval stage) physical and temporal separation of developing foregut modules (dorsal=larval esophagus; ventral=juvenile feeding structures) allows larval feeding and rapid switch to carnivory. However, previous studies on neogastropods with direct development (no feeding larval stage) did not identify foregut developmental modules. Thus, I investigated foregut development in two predatory, direct developing neogastropods: Nucella lamellosa and N. ostrina (Muricoidea), using histological sectioning, 3D reconstructions, TEM, and SEM. In both species, I showed evidence for dissociable dorsal and ventral foregut developmental modules. In N. lamellosa, the two modules were physically separate, although they were not separate in N. ostrina. My results reconcile differences in previous descriptions of foregut development between neogastropods with indirect and direct development. / Graduate