Geographical distribution of Poiycelis (Seidlia) auricuiata in Japan

Polycelis (Seidlia) auriculata is endemic to mountain districts of Japan, from the central part of Honshu to the area of the Daisetsu Mts of Hokkaido. In northern Japan, it sometimes occurs in cold-water biotopes of lowland areas. The progenitor of P. auriculata appears to have been the oldest immigrant into northern Japan among the Japanese Polycelis species, entering through a northern route as a pre­ glacial faunal element. P. auriculata now shows a discontinious distribution in northern Japan. By vir­ tue of its geographical and vertical distribution, ecological niche, variation in anatomy of the copulatory apparatus, and cytodemes, this species appears to be in the process of transformation.