W. J. CROZIER 591 



turns bright pink at pH = 5.6 ±; cf. Crozier, 1916); but the color 

 of the gills is independent of that of the other tissues of the animal, al- 

 though due to the same pigment. 



SUMMARY. 



The branchial plumes of Chromodoris respond, by contraction, to a 

 decrease in light intensity. This response is obliterated by high tem- 

 peratures (above 32°) and by direct sunlight, and is possible only 

 within a limited range of alkalinity of the sea water. A concealing 

 retraction of the whole gill-crown is reflexly determined by the self- 

 contraction of the individual plumes under "optimal" conditions of 

 light, temperature, and alkalinity. This protective response of the 

 branchiae is superimposed upon their simple system of fundamental 

 activities (protrusion, retraction) apparently concerned with regulat- 

 ing the respiratory exchange of the nudibranch. 



BIBLIOGRAPHY. 



Carey, L. R., 1917, Studies on the physiology of the nervous system of Cassiopea 

 xamachana, Carnegie Institution of Washington, Publication 251, 121-170. 



Crozier, W. J., 1916, Cell penetration by acids. II, /. Biol. Chem., xxvi, 217; 

 1919, Further contributions upon the natural history of Chromodoris zebra; 

 the question of adaptive coloration, Anat. Rec., xv, 349. 



Crozier, W. J., and Arey, J. B., 1919, Sensory reactions of Chromodoris zebra, 

 J. Exp. Zool., (in press), abstracted in Anat. Rec, 1919, xv, 345. 



McClendon, J. F., Gault, C. E., and MulhoUand, S., 1917, The hydrogen ion 

 concentration, CO2 tension, and CO2 content of sea water, Carnegie Institu- 

 tion of Washington, Publication 251, 21-69. 



