The Photodynamic Action of Light on Hydra.

Hydra were known to react to light as early as 1744 when Trembley first described their movement towards a light source. This migration of the animal was then further elucidated by Wilson in 1891, who found that Hydra viridis and fusca are maximally sensitive to blue light. Haug ( ' 3 3 ) found that Hydra will react to a light stimulus by contracting first and then orienting; and Rushforth, Burnett and Maynard ('63) studied the contraction response in Hydra pirardi, the large Belgian species. Feldman and Lenhoff ('60) studied phototaxis in the animal and suggested the presence of photoreceptors. The present work is an analysis of the influence of light intensity and wavelength on the contractility of Hydra pirardi employing in part the methods of study of Hecht ('20-'22) on the clam, Mya. It provides some evidence on the nature of light reception in Hydra and discusses the results in terms of known physiological functions.