1. The adult stage of Polystoma integerrimum feeds on blood which is digested by a combination of intraluminar and intracellular processes, haemolysis and partial digestion in the gut lumen being followed by absorption and completion of digestion by the cells of the gastrodermis.

2. The end products of intracellular digestion are the pigment haematin and a crystalline substance identified as haematoidin, which is chemically identical with vertebrate bilirubin, and these are passed back into the lumen to be eventually voided to the exterior.

3. It is probable that the expulsion of haematin from the gut cells results in a simultaneous release of enzymes, traces of which persist in the lumen to effect the early extracellular stages in the digestion of the next meal, so that intraluminar digestion in Polystoma appears to be an incidental extension of the primitive intracellular process arising from the blood-feeding habit and the consequent need to eliminate residues of haemoglobin breakdown from the gastrodermis.

4. Since haematin formation has been described in certain other trematodes it is suggested that their digestive processes probably follow a course similar to that described for Polystoma but an alternative method must exist within the class since other blood feeders such as Haplometra and Gorgodera do not form this pigment.