The simple tail of chordates: phylogenetic significance of appendicularians.

Appendicularia comprises a group of pelagic tunicates that retain the tail throughout their life without exhibiting the drastic metamorphosis seen in ascidians or doliolids. They are known to possess a simple body architecture that is comparable with that of other chordates. Recent phylogenetic studies suggest that appendicularians represent a sister group of the clade of other tunicates. Very recently, two independent research groups reported molecular-based approaches to the appendicularian development. We review here some general descriptions and results of recent analyses on the anatomy and developmental biology of appendicularians, focusing upon their simple tail architecture. We emphasize future possibilities for a comprehensive understanding of the divergent patterns in lifestyle of tunicates as well as for investigating the phylogenetic novelty and innovation of chordates such as the tail.