The pharynx (foregut) of acoel flatworms will be investigated with special focus on the cellular ultrastructure as
detected by transmission electron microscopy, and on the arrangement of pharynx musculature, as visualized by
fluorescence staining and confocal laser scanning microscopy. Additionally, 18S rDNA-sequences will be analyzed
to evaluate phylogenetic affinities of the selected species to each other and to other acoels.
Acoels are small, aquatic (predominantly marine), soft-bodied worms. Due to their apparent simplicity, they have
repeatedly been in the center of interest in discussions about the starting point of a phylogenetic tree of triploblastic
animals. In the traditional view, the Acoela is one of the major clades of the Platyhelminthes but recently it has
been suggested that they are not related to other flatworms but occupy a unique position as basal bilaterians.
Four species were selected (Paratomella rubra, Myopea "callaeum", Proporus bermudensis, Hofstenia giselae) to
represent the wide range of pharynx diversity within the Acoela, from a simple mouth without distinct elaboration
of a pharynx to a large pharynx bearing clusters of glandular cells. Based on the results on ultrastructure and
muscle patterns, the feasibility of the detected features as useful characters for acoel systematics and for the
reconstruction of acoel phylogeny will be evaluated. The probability of an independent evolutionary development
of pharynges in the investigated species will be discussed. Moreover, the results will be compared with published
data on other turbellarians and acoels, the homology of the diverse foregut configurations termed "pharynx
simplex" will be evaluated, and the most primitive state of the pharynx in acoels will be discussed..