The nature of the stream bottom is a very important factor with regard to animal communities and is to be discussed in terms of the particle size which a given current will move off the bottom. The stony bed of the riffles is stable and thus it may inhabit many benthic animals, while the sandy bed tends to shift, more or less, whenever the current increases somewhat above the normal, and the result is that conditions for animal life are more rigorous than on a stable bottom. Examples studied in a stream, Yudehara-gawa, Nosegawa-mura, Nara Prefecture are described as to the community of the stony bed (Table 1) and that of the sandy bed (Table 2). Various conditions relating to the bottom problem are discussed.
