406 F. W. GAMBLE AND FREDERIOK KEEBLE. 



iu illumination beliind tlie plate and below it is not great, no 

 reaction occurs. Here the stimulus of gravity dominates that 

 due to light. In the reflex groundwork of nervous activity 

 something akin to the phenomenon of attention in psychic life 

 already exists, just as in the phenomenon of after-effect, 

 especially in that peculiar after-effect which produces rliythm, 

 we have the reflex analogue of psycliic memory. 



6. Rheotropism, Thigraotropism : Application of 

 the Tropisms. 



Beside being influenced by light, gravity, and heat, the 

 movements of Con vol utaare also conditioned by the strength 

 of the water current and by the nature of the surface on 

 which the animal rests. These influences, as well as those 

 of light, gravity, and heat, we consider in relation to the 

 habits of the animal in the next section. 



We will conclude by pointing out in what ways a know- 

 ledge of the tropisms above described is useful iu the 

 manipulations required in an investigation of Convoluta. 



To obtain plentiful supplies of eggs, we make use of the 

 positive phototropic reaction. A great number of mature 

 Convoluta h collected in a large dish placed opposite a 

 window. The animals move up to the light; at nightfall the 

 dish is turned round. In the morning the animals all cross 

 to the lighted side, and the egg-capsules deposited at night 

 may be picked out free from parents. 



Again, young Convoluta are only seen with difficult}^ 

 owing to their minuteness. We prepare a sheet of white paper 

 and lay on it a black cloth, and so rest the dish containing the 

 young animals that the greater part of it is on the black 

 background. The animals collect on the part of the dish 

 above the white ground. There, however, they are all but 

 invisible, so that the dish must be gently turned till the part 

 previously on the white is on the bla(;k ground, against 

 which the animals are easily seen. To transfer a young 

 Convoluta from one vessel to another is difficult enough till 

 its geotropism is pressed into service. The animal is lifted 



