On the Distribution of Freshwater Turbellaria in the Aberystwyth District, with Especial Reference to Two Ice-Age Relicts

Abstract

The area includes 3 spp. of peculiar interest: Planaria albissima, previously recorded only from Bohemia, but very abundant in this district; P. alpina and Polycelis cornuta, glacial relicts surviving on the high Alps and in cold stenothermous mountain waters in the Harz, Taunus, etc. Both spp. occur in Cardiganshire streams almost down to the tidal limit; neither has been found above the 1000-ft. contour line. Geological history of the area and modifications in physiology and habit whereby the life cycle is adapted to a temperate climate are advanced as explanations.