A HISTOCHEMICAL STUDY OF DIGESTION AND DIGESTIVE ENZYMES IN THE RHYNCHOCOELAN LINEUS RUBER (O. F. MÜLLER)

¹ Department of Zoology, The University of Leeds, England

1. Digestion in the rhynchocoelan Lineus ruber is both extracellular and intracellular. The extracellular phase is entirely proteolytic and is brought about by an endopeptidase acting in an acid medium. The semi-digested food is then phagocytosed and digestion is completed in the second, intracellular phase by exopeptidases, lipases and presumed carbohydrases, all operating in an alkaline medium.

2. The following enzymes have been located and identified by histochemical methods: carbonic anhydrase, a cathepsin C type protease (endopeptidase), leucine aminopeptidase (exopeptidase), lipase and alkaline phosphatase.

3. Carbonic anhydrase occurs in acidophil gland cells in the buccal cavity and foregut. It is believed to be associated with production of acid used to kill the food and provide a suitable medium for the extracellular phase of digestion.

4. The cathepsin C type protease is produced by gland cells in the gastrodermis and is discharged into the gut lumen to bring about the initial extracellular proteolysis.

5. Leucine aminopeptidase is produced within the phagocytic cells of the gastrodermis when food vacuoles are present and is concerned in completion of protein digestion.

6. Lipase, identified as "true lipase" homologous with mammalian pancreatic lipase, is formed within the phagocytic cells at the same time as leucine aminopeptidase and attacks the fat content of the food.

7. Alkaline phosphatase is present in an active form in the gastrodermis and appears to he concerned with phagocytosis. The amount present increases when intracellular digestion occurs and this increase is apparently associated with secretion of aminopeptidase and lipase into the food vacuoles.