ULTRASTRUCUTURE OF 'TURBELLARIA' (PLATYHELMINTHES) BY

The systematics of turbellarian platyhelminths (also known as free-living flatworms) has proven difficult since few taxonomically useful characters can be discerned in them. Generally, features of the reproductive tract, observed through conventional light microscopy, provide key taxonomic characters. Through the newer techniques of electron microscopy and fluorescence microscopy, new characters are emerging that provide better clues to the phylogenetic relationships of these animals. We have applied both of these microscopies to representatives of two groups of turbellarians whose phylogenetic positions are uncertain and controversial, the Acoela and the Genostomatidae. Because fluorescence microscopy of phalloidin-labeled acoel turbellarians has provided new taxonomically relevant characters in the arrangement of muscles, we studied another feature that has emerged in such phalloidin-labeled preparations, namely sensory receptors. By correlating electron micrographic images and fluorescence images of Convoluta pulchra, we determined that these structures are sensory receptors with a central cilium surrounded by a collar o