The evolution of anural larvae in molgulid ascidians

Ascidians are urochordates, marine invertebrates with non-feeding motile ch ordate tadpole larvae, except in the family Molgulidae. Urodele, or tailed, Molgulids have typical ascidian char-date tadpole larvae possessing fails with muscle cells, a notochord, and a dorsal hollow nerve cord. In contrast , anural (or tail-less) Molgulids lack a ta ii and defining chordate featur es. Molecular phylogenies generated with 18S and 28S ribosomal sequences in dicate 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 evo lution of tail-less ascidians.