Pharyngeal and Gastrodermal Ultrastructure of Prorhynchus stagnalis Schultze, 1851 (Turbellaria,Lecithoepitheliata)

The digestive tract of a freshwater Lecithoepitheliata turbellarian, Prorhynchus stagnalis, has been studied at the ultrastructural level. The buccal tube connecting the mouth opening to the pharynx is lined by an insunk epithelium, and is provided with two kinds of secretory cells. The bulbous pharynx itself, typical of the genus, is a highly muscular organ, also lined internally by an insunk epithelium; it also bears at least two different types of gland cells. Externally the pharynx is enveloped by a thin, flat epithelium. The gastrodermis consists of phagocytes and Minotian gland cells, as typical for most Tricladida. No ciliary covering was observed along the whole digestive system; instead, phagocytic cells have long microvillar projections. Preliminary attempts to follow the digestive process using external markers and cytochemical reaction for acid phosphatase were successful in demonstrating endocytic activity in the phagocytes. These data, besides being a contribution to morphology and systematics, also speak in favour of the general digestive theory of Turbellaria proposed by Jennings and serve as a starting point for experimental studies on intracellular digestion within the Lecithoepitheliata.