"The first five papers were prepared at the zoological laboratory of the University of Michigan and were submitted to the Carnegie Institution for publication August 1, 1903. To the third paper some additions were made in February, 1904. The sixth and seventh papers were prepared at the Naples Zoological Station, while the writer was acting as research assistant of the Carnegie Institution, and were transmitted for publication in January and March, respectively, 1904."--Pref "Literature cited": p. 253-256 Reactions to heat and cold in the ciliate infusoria.--Reactions to light in ciliates and flagellates.--Reactions to stimuli in certain rotifera.-- The theory of tropisms.--Physiological states as determining factors in the behavior of lower organisms.--The movements and reactions of Amba.--The method of trial and error in the behavior of lower organisms extracted picklist