Morphology and ultrastructure of the pharynx in Solenofilomorphidae (Acoela)

Abstract

The homology of pharynges within the mostly pharynx-less Acoela has been a matter of discussion for decades. Here, we analyze the pharynges of three members of the Solenofilomorphidae, Myopea sp. and two species of the genus Solenofilomorpha, by means of light and transmission electron microscopy. Special focus is placed on the ultrastructure of the pharyngeal musculature, epidermis surrounding the mouth, pharyngeal epithelium, and junction with the digestive parenchyma. The main goal of this study was to evaluate the usefulness of certain characters for broader comparisons within the Acoela. Among the three species, 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. The arrangement of pharyngeal muscles and their connection with body wall musculature, ultrastructure of receptor cells, and morphology of a nonciliated glandular region in the posterior pharynx, in contrast, appear to be characteristic of the family Solenofilomorphidae and thus of predominant interest for comparisons with other acoel families.

Structure and evolution of the pharynx simplex in acoel flatworms (Acoela)

“…Species of Solenofilomorphidae have been examined in detail at histological and ultrastructural levels (Crezée, 1975;Todt and Tyler, 2006). A number of characters, such as the position of the mouth, presence and elaboration of sphincter muscles, and the presence of pharyngeal glands and epitheliosomes in the pharynx epithelium have been found to be species-or genus-specific, while the arrangement of pharyngeal muscles, the ultrastructure of receptor cells, and the presence of a nonciliated glandular region in the proximal Fig.…”

“…G. Sterreria psammicola. pharynx appear to be characters generally present in the family (Todt and Tyler, 2006). The pharynx is a ventral pharynx simplex sensu Riedl (1954).…”

“…The pharynx length is about 1/20 of the entire body length in M. callaeum and even less in the more elongate S. crezeei. The pharynx of both species is lined by a densely ciliated epithelium distinct from the surrounding epidermis (for details see Todt and Tyler, 2006). M. callaeum has long-necked pharynx glands opening laterally into the distal part of the pharynx.…”

“…Because of noticeable structural differences among pharynges and due to their scattered occurrence within the acoels there are conflicting views concerning their homology. Ax (1961) sees all acoel pharynges as derived from a single primitive pharynx simplex similar to that present in platyhelminth turbellarians (see also Tyler, 2001), whereas most other authors consider an independent origin as more plausible (Riedl, 1954;Karling, 1974;Crezée, 1975;Doe, 1981;Rieger et al, 1991;Todt and Tyler, 2006).…”

“…There are considerable variations though in histology, recruitment of pharynx muscles from the body wall, and layering of muscles. At least some of these features are constant enough among closely related acoel taxa to be used to characterize families or orders (e.g., Steinböck, 1966;Crezée, 1975;Hooge, 2001;Todt and Tyler, 2006). Representa-tives of Paratomellidae, however, often considered as the most primitive acoel taxon, lack a pharynx, as do most of the more derived acoel taxa and many Nemertodermatida (Rieger et al, 1991;Sterrer, 1998).…”