Studies on the Physiology of Uca Red Chromatophores

1. The red chromatophores of Uca pugilator, when other factors are equal, exhibit extensive responses to background. They disperse their pigment upon a black background, and concentrate it upon a white one. 2. Under conditions of constant darkness the red chromatophores show a striking persistent daily rhythmicity, dispersing their pigment by day, and concentrating it by night. 3. Extracts of sinus glands, and all of the major portions of the central nervous system, possess strong activity upon the red chromatophores. All portions exhibit two types of activity due to the possession of two principles. One of the activities is red-concentration and the other, red-dispersion. 4. The action of the red-concentrating principle can dominate the response when there is, concurrently present, high concentrations of both red-dispersing and red-concentrating principles. 5. The red-concentrating activity of all extracts is of much shorter duration than is the red-dispersing activity, the latter often lasting 3 to 4 times as long as the former. 6. Evidence is presented which strongly suggests that the black-dispersing and red-dispersing actions are due to one and the same principle.