ABSTRACT
The behaviour of Planarians belonging to a variety of species has been shown by a number of workers to depend largely upon the action of external stimuli—mechanical, chemical, light, heat, contact, etc.—inducing responses usually described as “tropisms,” which, while subject to some slight individual variation, remain on the whole constant in character for stimuli of the given degree of intensity. Observations have been made on reactions of this type in certain species occurring within the Aberystwyth district of Cardiganshire (Carpenter, 1926, 1927), with the special object of determining how far, if at all, such responses may assist in determining the peculiar ecological distribution of the species in question; the results seem worthy of a brief separate consideration, as they show that there may be considerable diversity of reaction between even closely allied species.
