70 PLANT-ANIMALS [CH. 



correlated with limited food supply. Now it has 

 been shown definitely in the case of other animals, 

 e.g. the caterpillars of Porthesia, that they are only 

 positively phototropic so long as they are not fed. 

 If this holds good for Copepods, their response to 

 increased carbon-dioxide becomes at once intelligible 

 on the mneme or associated stimulus hypothesis. 

 Thus hunger affects the tone or physiological state in 

 such a way that the Copepods respond to light by 

 directive movements whereby food supplies become 

 available. The movement brings the animals from a 

 part of the water which contains a maximal amount 

 of carbon-dioxide to a part where, thanks to the 

 presence and activity of the green algse, the food 

 sought by the Copepods the water is not fully 

 saturated with carbon-dioxide. When the animal 

 encounters carbon-dioxide conditions which are 

 normally associated with hunger conditions, it takes 

 the hint and photo tropes just as though it were 

 hungry. For a hungry man, a cook-shop window has 

 an irresistible attraction, whereas to the well-fed 

 person it may offer no seduction, or even be 

 repulsive : nevertheless, " si par impossible " the 

 odour which emanates from it is very agreeable, the 

 well-fed may deign to sniff. 



What internal changes, chemical or other, resulting 

 from the prolonged light-exposure of C. roscoffensis 

 on the beach, give the signal for its dismissal from 



