636 F. W. GAMBLE AND F. W. KEEBLE. 



mains relatively high during the nighty the vertical movements 

 of the Plankton still continue. By periodicity is meant that 

 rhythmic use breeds rhythmic habit ; that by recurrent 

 change of light or other stimulus the organism comes to 

 manifest the phenomenon of movement or other change, to 

 lapse again to its former condition, and to repeat this sequence 

 at resrular intervals althouofh the stimulus be withheld. 



In the case of the colour- changes of Hippolyte, change 

 of light-intensity is the efficient stimulus. By this stimulus 

 the red and yellow pigments may be expanded. By the 

 stimulus resulting from a lowered light-intensity these may be 

 retracted and replaced by blue. If these changes are a func- 

 tion of the time, i. e. if they are truly periodic, they should 

 exhibit themselves when light-conditions are constant. This 

 they do. We will not stay to discuss whether a mental pic- 

 ture of the process can better be obtained by likening the 

 phenomenon of periodicity to "memory" (see Darwin and 

 Pertz., ' Annals of Botany,^ vi, p. 262), or to a persistent after- 

 effect of precedent stimuli, but proceed at once to give evi- 

 dence of the fact. This evidence is derived from experi- 

 ments on the effects of continued darkness and continued 

 light on the prawns. 



Effect of Continued Darkness (Table IX).— In each 

 of four " air-circulators " three specimens of three similar 

 colour-forms were placed. They were fed with pieces of 

 other prawns. The eyes of the prawns in two of these flasks 

 (B and D) were amputated. Flasks A, B, and D were covered 

 (by double folds of close black cloth) ; C, a control, was 

 exposed to the light. The experiment lasted from Septem- 

 ber 3rd, 10.30 a.m., till September 7th. During each exa- 

 mination necessary for recording, the black cloths were 

 lifted for the shortest possible time — usually about fifteen 

 seconds. In the afternoon of September 8rd, the prawns in 

 the covered vessels showed distinct change in the direction 

 of transparency and greenness, one or two being nocturnal. 

 The prawns in C, the exposed vessel, showed no colour- 

 change. In the evening all in the control and in the covered 



