The rhipidoglossan radula: Formation and development in Margarites helicinus Phipps, 1774 (Trochoidea, Vetigastropoda).

The gastropod radula exhibits exceptional morphological variability. Despite this enormous diversity, the main patterns of synthesis of the teeth and radula membrane, characteristic of different groups can be identified. The rhipidoglossan radula contains numerous teeth in each transverse row and was found in phylogenetically distant groups of Gastropoda. Studying radula formation through stages of ontogeny in species with this type of radula is important in determining the main patterns of radula formation in gastropods. We studied the formation of the radula during development of one species of trochid vetigastropod, Margarites helicinus, using light and electron microscopy as well as confocal laser scanning microscopy. The adult radula is formed in the blind end of the radular sac, which bifurcates into two horns in vetigastropods. The numerous marginal teeth are synthesized in these horns while the central and lateral teeth form in the region where the horns fuse. This morphology of the formation zone appears to be a common pattern for all vetigastropods. The juvenile radula of M. helicinus consists of seven teeth per transverse row and its formation in the radular sac differs significantly from that in the adult. In the juvenile, the formation zone of the radular sac is not split into two horns, and the teeth and radular membrane are synthesized by relatively few, uniform cells. This organization of the larval radular sac is thought to represent a widely occurring larval pattern potentially present in all groups of gastropods. It is associated with early formation of all organs by few cells rather than representing a phylogenetic trait.