Evolution of body-wall musculature in the platyhelminthes (Acoelomorpha, Catenulida, Rhabditophora)

In an effort to understand the phylogeny of the Platyhelminthes, the patter ns of body-wall musculature of flatworms were studied using fluorescence mi croscopy and Alexa-488-labeled phalloidin. Species of the Catenulida have a simple orthogonal gridwork of longitudinal and circular muscles. Members o f the Rhabditophora have the same gridwork of musculature, but also have di agonal muscles over their entire body. Although a few species of Acoelomorp ha possessed a simple orthogonal grid of musculature, most species typicall y have distinctly different patterns of dorsal and ventral body-wall muscul ature that include sets of longitudinal, circular, U-shaped, and several ki nds of diagonal muscles. Several distinct patterns of musculature were iden tified, including 8 patterns in 11 families of acoels. These patterns have proven to be useful in clarifying the phylogeny of the Acoelomorpha, partic ularly with regard to the higher acoels. Patterns of musculature as well as other morphological characters are used here for revisions of acoel system atics, including the return of Eumecynostomum sanguineum (Mecynostomidae) t o the genus Aphanostoma (Convolutidae), the revision of the family Childiid ae, and the formation of a new family, Actinoposthiidae. J. Morphol. 249:17 1-194, 2001. (C) 2001 Wiley-Liss, Inc.