the microscopic organisms that inhabit moist sand along rivers and lakes. Psammon is most abundant at depths 0.5–1 cm from the soil surface. It consists of many diatomaceous, protococcal, and blue-green algae; it also includes infusorians, testaceans, rotifers, tubellarians, roundworms, oligochaete worms, chironomid larvae, and other animals that can move in the interstices of sand. Sometimes microscopic organisms that live in littoral marine sands are also called psammon.