76 LIFE IN THE SEA [CH. 



contain chlorophyll in the tissues are also affected. 

 Those which have eyes will probably perceive light 

 changes in the way we do, but even if eyes are 

 absent there is always the possibility that light 

 changes can be perceived by means of the skin. 

 Intense light radiation may even kill some of the 

 lower organisms, and in ourselves certain well-known 

 curative effects depend on some such reaction on 

 the part of diseased tissues. Almost all animals 

 which can be kept in aquaria respond by directed 

 movements to changes in the intensity of the light 

 falling on them. In the cases of the fishes and 

 higher Crustacea these movements are usually away 

 from the source of light, but in the cases of the lower 

 organisms they may be either towards or away from 

 the light. Plaice and flounders in aquaria usually 

 prefer the darker parts ; and in the sea they lie on 

 the bottom during the day, but at night they may 

 swim up towards the surface. Crustacean larvae 

 usually swim to the source of light when the other 

 conditions are normal. The abundance of animal 

 plankton in the sea is affected by the intensity of 

 light in that the organisms move up and down from 

 stratum to stratum of water. We usually get a 

 different kind of plankton during the day from that 

 we get at night, and even the amount of cloud in 

 the sky will make a difference. 



If a species of plant or animal can possibly 



