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THE AMERICAN NATURALIST [Vol. LVI 



house for four years. In the autumn of the first year 

 it became very fat and stored a large quantity of food in 

 its burrow. About December 1, it went into its burrow, 

 closed the opening, and remained underground for 119 

 days. The following autumn the spermophile behaved in 

 a similar way but remained underground for only 28 days. 

 It did not hibernate during the two years following. This 

 animal had an established seasonal metabolic rhythm that 

 was correlated with seasonal environmental changes, but 

 the rhythm had a physiological basis for it persisted when 

 appropriate environmental stimuli were not present. 



The rotation of the moon about the earth introduces 

 certain rhythmic variations into the earth environment to 

 which animals respond in adaptive ways. Such responses 

 are of course not due directly to the moon as such, but 

 to effects of the moon's motion on matter belonging to the 

 earth. The famous Palolo worm and various other ma- 

 rine annelids come from their hiding places to spawn only 

 during certain phases of the moon. In these worms the 

 eggs do not ripen except when the moon is new or full ; the 

 internal activities respond to outside changes, chiefly 

 referable to tidal variations, and a physiological rhythm 

 is established. 



The earth rotates on its axis and thus the animals on 

 its surface are subjected to alternating light and dark. 

 Animals readily respond to this short-period rhythmical 

 change. Every one is familiar with nocturnal and diurnal 

 animals. They are adapted to rhythmical environmental 

 changes to such a degree that they may keep on respond- 

 ing periodically when the environment does not change. 

 Keeble and Gamble^^ have described an interesting shrimp 

 {Hippolyte rarinns) that has day and night color phases. 

 During the day this slirinip matches the background on 

 which it rests with a higli degree of accuracy, assuming 

 quite a variety of colors and patterns. At night it turns 

 green, regardless of its background. When kept contin- 

 uously in light it undergoes rhythmic color changes at 

 about the time periods that correspond to day and night 



11 Phil. Trans., London, 1904. 



