FURTHER STUDIES ON FEEDING AND DIGESTION IN TRICLAD TURBELLARIA

¹ Department of Zoology, The University of Leeds, England

1. Feeding and digestion in two species of triclad Turbellaria, one aquatic, the other terrestrial, have been investigated by histochemical methods to locate and identify a selection of the enzymes concerned in the two processes.

2. In both species the pharynx possesses acidophil gland cells which produce endopeptidases of the cathepsin C type, and the available evidence indicates that these are used to assist the pharynx in its penetration of the prey's body wall, and the subsequent disruption of the body contents prior to ingestion.

3. Food entering the gut is attacked by extracellularly-acting endopeptidases, similar to those produced in the pharynx, and originating from the sphere cells of the gastrodermis. This intraluminar digestion continues and extends break-up of the food initiated by the pharynx, and serves to make the bulk of it available for phagocytosis and intracellular digestion.

4. Columnar cells of the gastrodermis phagocytose food from the gut lumen and digest it within vacuoles containing enzymes secreted from the cytoplasm in a definite sequence.

5. The contents of the food vacuoles are attacked first by endopeptidases similar to those secreted into the gut lumen and acting in an acid medium of pH 5.0.

6. Endopeptidase activity within the vacuoles is eventually replaced by exopeptidases, such as leucine aminopeptidase, plus lipase and unidentified carbohydrases, all acting in a slightly alkaline medium of pH 7.2.

7. Secretion of the various intracellular enzymes involves the appearance of phosphatases in both the cytoplasm and the vacuoles of the columnar cells. Acid phosphatase appears to be concerned with the secretion of endopeptidase in the first stage of intracellular digestion and alkaline phosphatase with the production of the other digestive enzymes.