In three species of terrestrial isopods the rate of ingestion of litter decreases but the efficiency of assimilation increases with rising concentrations of copper in the food. 2) When feeding on natural litter with a low concentration (20 ppm) of copper at 22°C, all three species of isopods tested lose more copper through their feces than they ingest. When fed artificially enriched litter, the efficiency of assimilation increases strikingly so that at the highest concentration of copper tested (5200 ppm) between 80 and 90% of the ingested metal is assimilated. 3) At 8°C copper balance of isopods never becomes negative, not even with the lowest concentration of copper in the food. 4) Both types of experiment indicate that the more slowly food passes through the gut the more efficient is the digestive system of the isopods in extracting copper from it. 5) Isopods are capable of digesting even tightly bound copper during one single passage of food through the gut. However, they are unable to resorb more copper than they lose unless the food is enriched with soluble copper or the rate of food passage through the gut is slowed down. This deficiency in the process of resorption is likely to be the reason for coprophagy in this group of animals. 6) When feeding on litter not in an advanced state of decay, the isopods arehyperphagous. This may have the-teleological-advantage that organic material is more rapidly converted into feces by the isopods and thus prepared for the attack of microorganisms.
