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a separate organism (individual) adapted in the course of evolution to dwelling in a definite environment (biotope). A distinction is usually made between aerobionts (dwellers on land and in the air), hydrobionts (aquatic organisms), geobionts (dwellers in the soil), and parasites (dwellers in other organisms). Organisms capable of living under different conditions are called eurybionts. Organisms living only under narrowly restricted conditions are called stenobionts. Examples of stenobionts are psammobionts (dwellers in sand), petrobionts (organisms living on rocky soil), and bothrobionts (dwellers in holes).