THE POSITION OF THE ARTHROPODA IN THE PHYLOGENETIC SYSTEM

Traditionally, Panarthropoda (Euarthropoda, Onychophora, Tardigrada) a re regarded as being closely related to Annelida ina taxon Articulata, but this is not supported by molecular analyses. Comparisons of gene sequences suggest that all molting taxa (Panarthropoda, Nematoda, Nema tomorpha, Priapulida, Kinorhyncha, Loricifera) are related in a monoph yletic taxon Ecdysozoa. An examination of the characters supporting Ar ticulata reveals that only segmentation with a teloblastic segment for mation and the existence of segmental coelomic cavities with nephridia support the Articulata, whereas all other characters are modified or reduced in the panarthropod lineage. Another set of characters is pres ented that supports the monophyly of Ecdysozoa: molting under influenc e of ecdysteroid hormones, loss of locomotory cilia, trilayered cuticl e and the formation of the epicuticle from the tips of epidermal micro villi. Comparative morphology suggests Gastrotricha as the sister grou p of Ecdysozoa with the synapomorphies: triradiate muscular sucking ph arynx and terminal mouth opening. Thus there are morphological charact ers that support Articulata, but molecular as well as morphological da ta advocate Ecdysozoa. Comparison of both hypotheses should prompt fur ther thorough and targeted investigations. J. Morphol. 238:263-285, 19 98. (C) 1998 Wiley-Liss, Inc.