Ascidians are urochordates, marine invertebrates with non-feeding motile chordate tadpole larvae, except in the
family Molgulidae. Urodele, or tailed, Molgulids have typical ascidian chordate tadpole larvae possessing tails
with muscle cells, a notochord, and a dorsal hollow nerve cord. In contrast, anural (or tail-less) Molgulids lack a tail and defining chordate features. Molecular phylogenies generated with 18S and 28S ribosomal sequences indicate that Molgulid species fall into at least four distinct clades, three of which have multiple anural members. This refined
and expanded phylogeny allows careful examination of the factors that may have influenced the evolution of tail-less
ascidians.