Dugesia tigrina ( syn. Euplanaria tigrina, Planaria maculata) reproduces in nature in two different ways ; sexually, by egg capsules, and asexually, by fission. This fact was first observed by Curtis ( '02, p. 556) who studied the life cycle of D. tigrina in the vicinity of Falmouth, Massachusetts. Curtis reported that the animals showed different life cycles in different localities. I n one locality, they reproduced exclusively by fission, in another only by egg capsules, and in two others both by fission and by egg capsules at different seasons. These observations have been repeatedly confirmed in the same localities through subsequent collections (Curtis and Schulze, '24, p. 105).
Recently, animals from Curtis's localities were cultured in the laboratory under various external conditions (Kenk, '37, '40). Their characteristic habits of reproduction proved to be, generally, independent of the physico-chemical properties of the environment, and therefore to be determined by their genetic constitution. The species appears to occur in at least two different physiological races, a "sexual" and an "asexual" or "agamic" race. The agamic animals reproduce in the laboratory entirely by fission, while the sexual form, under the same external conditions, develops genital organs in regular yearly cycles and produces egg capsules. Fission may occur in the sexual race at certain seasons, during sexual inactivity, but is neither a regular nor a frequent phenomenon.
'L. H. IIyman states that she knows of 110 cases where sexual worms are found in larger ponds and Iakes away from the shores in the vegetation of such places. In tho past summer ( J u l y and August, '40), Dr. F. E. Eggleton and the author collected sexual specimens of D. tigrina in Douglas Lake, Cheboygan County, Michigan, over f mile from shore, in a depth of 4 to 5 m., among Chara. Animals collected in the same season under stones along the shore were much larger, but all asexual. VOI. 170, pp. 249-251. 11122 planaires triclades paludjcoles.