THE RESPONSES OF SHRIMPS 137 



have to consider its nervous condition, which is a 

 function or outcome of a long succession of days and 

 nights. By turning its night into day and day into 

 night we find that for the first day or so of experiment 

 the change of tone still occurs at the appropriate hour. 

 Thus at nightfall, in spite of artificial light, the animal's 

 colour deepens into red, then to green, and finally to 

 blue ; its limbs, heart, and breath take on a new rhythm, 

 which endures for a night and then disappears. In 

 the same way, if we prevent the morning light from 

 making itself felt, the shrimp will still, for a day or so, 

 time its recovery from a nocturnal bout as though 

 informed of day's appearing. In fact, the nervous 

 system is the master, and for a while its innate perio- 

 dicity determines the rate of the animal's activity. 

 This result shows what a supremely important part 

 light and darkness play in animal economy. Con- 

 tinued bright light shocks it to stillness ; dim light 

 gives it the necessary suppleness of limb. 



But now, suppose this action worn down by the con- 

 stant effect of continued light or darkness; we have 

 then the shrimp in a more responsive mood. The 

 tone of its surroundings now influences its colour in 

 a more direct fashion. On light backgrounds it 

 quickly assumes a pale tint. On dark ones its red 

 and brown pigments produce a dark tone. If we 

 cover its eyes this faculty of sympathetic colour- 

 change is abolished. The brown colour persists under 

 all conditions of illumination, and we thus conclude 

 that the skin is influenced not so much directly by light 



