| Abstract: | Evolutionary changes in the development of shell-attached retractor muscles in gastropods are of fundamental importance to theories about the early evolution and subsequent diversification of this molluscan class. Development of the shell-attached retractor muscle (columellar muscle) in a caenogastropod has been studied at the ultrastructural level to test the hypothesis of homology with the post-torsional left retractor muscle (larval velar retractor) in vetigastropod larvae. The vetigastropod muscle has been implicated in the generation of ontogenetic torsion, a morphogenetic twist between body regions that is important to theories about early gastropod evolution. Two shell-attached retractor muscles develop sequentially in the caenogastropod, Polinices lewisii, which is a pattern that has been also identified in previous ultrastructural studies on a vetigastropod and several nudibranch gastropods. The pattern may be a basal and conserved characteristic of gastropods. I found that the first-formed retractor in larvae of P. lewisii is comparable to the larval velar retractor that exists at the time of ontogenetic torsion in the vetigastropod, Haliotis kamtschatkana. However, the post-metamorphic columellar muscle of P. lewisii is derived exclusively from part of the second-formed muscle, which is comparable to the second-formed pedal muscle system in the vetigastropod. I conclude that the post-metamorphic columellar muscle of P. lewisii, is not homologous to the larval velar retractor of the vetigastropod, H. kamtschatkana. |
