The vertical distribution of freshwater planarians in the Lake Doya-ko district, the south-western part of the Shikotsu-Doya National Park in South Hokkaido (Lat. 43°15′N. to Lat. 43°55′N. and Long. 140°35′E. to Long. 141°20′E.), is reported. The Lake Doya-ko district is characterized by volcanoes (Mt. Tarumae, Mt. Eniwa, Mt. Usu, Mt. Syowashinzan, Mt. Shiribeshi, Mt. Konbu, etc.) and three caldera lakes (Lake Doya-ko, Lake Kuttara-ko and Lake Shikotsu-ko). The highest peak, Mt. Shiribeshi (Mt. Makka'nupuri), is 1893 metres above sea level. There are many hot springs in this district. The Shiribetsu and the Osaru are main river systems of the area surveyed. The Shiribetsu River discharges into Iwanai Bay, the Sea of Japan. The Osaru River and the other many short rivers discharge into the Pacific Ocean. The surveys were made in August 4-5,1966. In the area surveyed, only two species of freshwater planarians, Polycelis sapporo (IJIMA et KABURAKI) and Polycelis auriculata IJIMA et KABURAKI, were found. Pol. sapporo was common in the stations both in the seashore and the mountainous districts. The altitude of the upper limit of distribution of this species is about 730 metres (inhabitable water temperature range, 6.4-16.5℃). Pol. auriculata was rather common in the cold-water stations in the altitude range from about 70 to 900 metres (inhabitable water temperature range, 5.9-13.2℃). The type of the vertical distribution in the area surveyed is shown as S-SA-A (S : Pol. sapporo ; A : Pol. auriculata).
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