1^^^] Gee: Behavior of Leeches 289 



minutes, however, they became quiet, entirely disregarding the 

 light. The leeches crawling freely about the dish were, however, 

 excited to greater activity, swimming rapidly about in the dish 

 and crawling up its sides. The light seemed to overcome their 

 normal geotactic reactions. Three and a half hours after the 

 beginning of the experiment the animals in contact with the 

 dish and stone were all in the same positions as at first. A half 

 dozen of the others were at rest on the sides of the dish, some 

 of them half out of the water. As many more were swimming 

 about in the dish or crawling on the bottom, while the remainder 

 were massed in clumps on the Ceratophyllum in the aquarium, 

 well up towards the top and in the most shaded portion of the 

 dish. Thus, light of the intensity used seems to overcome what 

 geotaxis the leech may have, the orienting effect of its rays 

 tending to carry the animal against the force of gravity. In 

 the case of the leeches underneath the stone, their thigmotactic 

 inclinations being satisfied, light of the intensity used had prac- 

 tically no effect. 



Sherrington (1911) says: "It is not usual for the organism 

 to be exposed to the action of only one stimulus at a time. It 

 is more usual for the organism to be acted on by many stimuli 

 concurrently, and to be driven reflexly by some group of stimuli 

 which is at any particular moment prepotent in action on it. 

 Such a group often consists of some one pre-eminent stimuliis 

 with others of a harmonious relation reinforcing it." The cases 

 of combined stimuli discussed in the preceding paragraphs afford 

 good examples of how well this principle applies in the leech. 

 In the case of the food juice and contact stimulation, we have 

 a prepotent stimulus in the food, and this reinforces the positive 

 reaction, which represents distinctly one of the feeding reflexes. 

 With light and contact combined, we have a prepotent stimulus 

 in the contact of the animal with the under surface of the stone 

 in the dish. These eases cited in the leech merely indicate the 

 significance of combined stimuli on the behavior of the animal, 

 and point to the fact that behavior represents a resultant action 

 to varied groups of concurrent stimuli. 



