Patterns of musculature as taxonomic characters for the Turbellaria Acoela

While turbellarians are generally assumed to have body-wall musculature con sisting routinely of longitudinal, circular, and diagonal fibers, members o f the Acoela examined by a fluorescence-microscopy technique specific for a ctin showed more complicated and distinctive arrangements of muscles, givin g promise for better delimiting taxa within this taxonomically difficult or der. Certain globose or tear-drop-shaped worms such as Convoluta pulchra an d species of Pseudaphanostoma, Mecynostomum, and Otocelis, showed a complex pattern in which muscles longitudinal in the anterior half of the body are diagonally across the posterior half; complex brushes of parenchymal muscl es that cross at the level of the statocyst and are postero-laterally also characterize these groups. The more elongate acoel Paratomella sp. was foun d to have musculature dominated by strictly longitudinal fibers and with re latively weak circular fibers and few fibers running diagonally to the body axis, yet the elongate mecynostomid Paedomecynostomum bruneum showed a cro ssing of antero-longitudinal fibers similar to that seen in the more globos e Mecynostomum sp. A distinctive looping of muscles around the mouth is see n in P. bruneum and the Anaperidae. Such similarities and differences in pa ttern of musculature promise to provide easily recognizable characters for taxonomy of the Acoela at levels ranging from species to family.