Homology of ciliary bands in spiralian trochophores

A number of hypotheses have been presented regarding the origins of the metazoans and, more specifically, the Bilateria. Using various phylogenetic analyses, characteristics have been mapped on phylogenetic trees to infer ancestral body plans and life history strategies of those ancestors. Many arguments on the evolution of the Bilateria are based on the presumed homology of certain characteristics of extant larva and adults, including various ciliated bands involved in feeding and locomotion. This article considers a recent study indicating that the second, downstream-collecting, ciliated band in the veliger larva of the gastropod mollusc, Crepidula fornicata, is actually derived from secondary trochoblasts (derived from second quartet micromeres), that normally form part of the prototrochal band found in other spiralian phyla (Hejnol et al. 2007). Despite previous arguments, these new findings suggest that the second ciliated band in the veliger larva is not homologous to the metatroch found in the trochophore larva of some other spiralians, such as the annelid, Polygordius lacteus. In the latter case, the metatroch was reported to be formed by a different set of lineage precursors (derived from third quartet micromeres) (Woltereck 1904). These findings have important implications for the interpretation of various hypotheses related to the evolution of metazoan phyla.