56 PLANT-ANIMALS [CH. 



menon of attention in psychic life may manifest 

 itself. 



Even though he may not be concerned with 

 problems of reflex action, the biologist who would 

 investigate the life histories or structure of such 

 animals as C. roscoffensis must pay some heed 

 to their tropistic behaviour, for on this knowledge 

 his successful manipulation of the living animals 

 depends. 



Thus, to obtain plentiful supplies of eggs we may 

 make effective use of the phototropic reaction. Large 

 numbers of mature C. roscoffensis are placed near a 

 window in a flat, glass dish containing sea- water. The 

 animals move up to the light. At nightfall, the dish 

 is turned round. The operation, if performed care- 

 fully, does not disturb the animals. They remain 

 throughout the night in that part of the vessel which 

 was nearest the light during the previous day. Cer- 

 tain of the animals deposit egg-capsules. In the 

 morning, the animals, responding to the directive 

 stimulus of light, cross over to the side turned 

 toward the window. The egg-capsules thus left 

 behind are readily visible to the eye and may be 

 picked out by means of a pipette. In this way, several 

 hundred egg-capsules may be obtained in the course 

 of a few days. 



Again, young C. roscoffensis are so minute that 

 they may be found only by a practised eye. Never- 



