Stach, T. and Kaul, S. 2011. The postanal tail of the enteropneust Saccoglossus kowalevskii is a ciliary creeping organ without distinct similarities to the chordate tail. —Acta Zoologica (Stockholm) 92: 150–160.
The postanal tail of chordates is one of the key characters in chordate evolution and it has been suggested to be homologous to the postanal tail of harrimaniid enteropneusts. We present electron microscopic data of the ontogeny of the postanal tail in the enteropneust Saccoglossus kowalevskii. The postanal tail develops as a ventral posterior allometric outgrowth with a ventral extension of the telotroch. Transmission electron microscopy of serial sections reveals the epidermal organization of the postanal tail with the exception of short, bilaterally symmetric extensions of the paired metacoels. The epidermis cells are connected by apical junctions, rest basally on the extracellular matrix surrounding the mesoderm, and possess a basiepidermal nerve net. The ventral cells in the postanal tail are multiciliated and used for creeping. Dorsal cells are monociliated with numerous microvilli. Two types of glandular cells are present among the epidermis cells. The mesoderm cells contain myofilaments. We were unable to detect anatomical structures similar to the ones present in the postanal locomotory tail of chordates, such as notochord, neural tube, or endodermal strand. Thus, results of our anatomical study do not support homology of the postanal chordate tail and the postanal tail of harrimaniid enteropneusts.