STUDIES ON STENOSTOMUM GRANDE CHILD, 1902 (PLATYHELMINTHES, CATENULIDA) - FINE-STRUCTURE OF THE DIGESTIVE-TRACT AND THE ENDOCYTOTIC ACTIVITY OF THE GASTRODERMIS

The digestive tract and its endocytotic activity in the catenulid Sten ostomum grande were studied by electron microscopy. The pharynx was ty pical of the simplex type. At the mouth, between the integumental epit helium and the pharyngeal epithelium proper, was a transition zone. Am ong the epithelial cells of this transition were monociliated sensory cells and the necks of bucco-pharyngeal secretory cells of two types. The pharyngeal epithelium proper was densely ciliated, with long cilia ry rootlets and mitochondria. It was surrounded by two layers of muscl es. The gastrodermis consisted of phagocytes and typical secretory Min otian cells. It was underlain by a delicate basal lamina and muscle fi bers. Distinctive of the phagocytes was the presence of differentiated cilia, cup-shaped mitochondria, and vacuoles with dense inclusions. M orphological differences between pharyngeal and gastrodermal cilia sug gest functional differences, Experiments using latex beads as tracers and the identification of acid phosphatase in cytoplasmic vacuoles poi nted to a high level of endocytotic and digestive activity in the phag ocytes. Our data demonstrate that the basic structure of the digestive tract in S. grande conforms well to that of other free-living platyhe lminths, but it does have ultrastructural peculiarities.