1919] EsterVy: Reactions of Various Plankton Animals 11 



mental work was done with the arthropod Idothea. He found that 

 there is a periodical change in the chromatophores corresponding to 

 day and night; the animals are darker by day than by night (p. 43). 

 Menke formulates the proposition (p. 57) that variation in light in- 

 tensity acts as a regulating mechanism. If external conditions are 

 kept constant the chromatophores contract and expand corresponding 

 to the usual periodicity of light and darkness. He was even able to 

 reverse the rhythm, so that the old rhythm was set aside, by lighting 

 the animals at night and keeping them in darkness by day. After 

 nine days of this they were kept in constant darkness, and for about 

 a week the pigment cells contracted by day and expanded at night. 

 Menke cites other facts that show that there is a periodicity of meta- 

 bolism : the animals are livelier at night and the heart beats twice 

 as fast as by day (pp. 64-66), and, furthermore, agents such as tem- 

 perature and acids affect the movements of the chromatophores. 



Such facts are applied by Menke (pp. 79-80) to the vertical migra- 

 tions of plankton animals. He notes that the heliotropism of such 

 forms has been shown to be reversed by temperature changes, which in 

 turn affect the chemical processes, and he says (p. 80) : "In weiterer 

 Verfolgung dieses Gedankenganges wird man zu den Schluss gedrangt, 

 class die primare Ursache Yertikalwanderung mancher planktoniseher 

 Organismen in Stoffwechselanderungen legt, und dass erst sekundiir 

 mit diesen chemischen Prozessen ein heliotropischer Stimmungsweehsel 

 verkettet ist. " The periodical migrations and the periodicity of chro- 

 matophore movements are occurrences of essentially like nature, con- 

 ditioned by periodicity in metabolism. The same agents that affect 

 pigment cells alter the heliotropism and in corresponding ways. Rais- 

 ing the temperature causes negative heliotropism in many animals and 

 it also brings about contraction of the pigment cells. Lowering the 

 temperature causes expansion of Chromatophores and reverses the 

 heliotropism to positive (p. 81). An interesting suggestion is made 

 (p. 82) as to the regulatory character of the migration. The osmotic 

 pressure of the body fluids is altered as the metabolic changes take 

 place, and the animals move into regions of corresponding pressure to 

 compensate for this. 



In brief, the conclusion reached by Menke, as given on page 88, is 

 that the movements of the chromatophores and those shown in the 

 diurnal migration are "autonomous," and that they parallel a peri- 

 odicity in metabolism which is probably characteristic of all living 

 matter. The fact that the metabolic rhythm follows the daily changes 



