A comprehensive report on the special conditions under which the torrential fauna lives and on the adaptations in this fauna. Nielsen maintains that competition has forced some animals into the torrential habitat and that a number of adaptations to withstand the currents have been evolved. Steinmann''s theory that flattening of the body is an adaptation to life in torrents is shown to be incorrect, or at least of little significance. Adaptations in feeding habits and their morphological counterparts are also shown. The paper contains 14 instructive schematic drawings.