Ultrastructural Observations on the Phagocytic Uptake of Food Materials by the Ciliated Cells of the Rhynchocoelan Intestine

Ultrastructural observations on the rhynchocoelan Lineus ruber confirm that the ciliated columnar cells of the gastrodermis are truly phagocytic, in that they engulf visible particulate material from the intestinal lumen, enclose it in food vacuoles and pass these back into the cell for subsequent intracellular digestion. 2. The cilia have the form and structure that is ubiquitous for these organelles throughout the animal kingdom and they are not concerned in any way with the phagocytic uptake of material from the gut lumen. 3. After the initial extracellular digestion, which renders the food semi-fluid and relatively homogeneous, the cell surfaces between the cilia develop pseudopodia-like outgrowths or lamellae which extend out into the gut lumen. The tips of two adjacent lamellae fuse to form a vacuole enclosing semi-digested food and the vacuole then moves back into the cell for completion of digestion. 4. Lamellae are only present when semi-digested food is present in the intestine and they are absent from the resting gastrodermis between meals. 5. Microvilli occur between the gastrodermal cilia and are believed to be concerned with absorption of simple soluble substances from the gut lumen. They also occur between the cilia of the epidermis and of the foregut. 6. A zone of alkaine phosphatase activity, not associated with any particular organelle, occurs in the extreme distal region of the columnar cells. Acid phosphatase activity can be demonstrated in lysosomes occurring in the basal regions of the cells.