TODT, C.*; TYLER, S.; Univ. of Maine, Orono; Univ. of Maine, Orono: Comparative studies on the pharynx simplex of Acoela

The homology of pharynges within the mostly pharynx-less Acoela has been a matter of discussion for decades, and still the basic question of whether a pharynx is a primitive trait within the Acoela and is homologous to the pharynx of other turbellarians is unanswered. Nemertodermatida, the sister group to the Acoela, lack a true pharynx, as does Paratomella, by some considered the most basal of acoels. Other candidates for this basal position, including Solenofilomorphidae, Hofsteniidae, and Proporidae, have well developed pharynges. We present the first comparative study of the pharynx simplex in these groups using transmission electron microscopy and fluorescence staining of musculature in the mouth and pharynx as well as conventional histological techniques. Among three species of the Solenofilomorphidae, characters relating to position of the mouth, presence and elaboration of sphincter muscles, presence of pharyngeal glands, and ultrastructure of epitheliosomes proved to be variously species- and genus-specific. For comparisons to other acoel families, arrangement of pharyngeal muscles and their connection to body-wall musculature, ultrastructure of receptor cells, and morphology of a non-ciliated glandular region in the posterior pharynx provided better insights. Our results show major differences of these characters among the studied species of Solenofilomorphidae, Hofsteniidae, and Proporidae. This confirms the great variability of pharynx morphologies in the Acoela and points to an independent evolution of pharynges within these families.