E. tigrina (=Planaria maculata Leidy) occurs in 2 physiological races: an asexual race, which reproduces exclusively by fission[long dash]fission being inhibited by both low and very high temp. and starvation; the sexual race, which periodically develops reproductive organs and lays cocoons. High temp. increases the breeding rate, low temp. and starvation decrease it. After the breeding period has ceased, the sex organs degenerate and fission may occur. Many fissioning spp. of planarians seem to have races differing in the habits of reproduction.