observations on hydra 599 



Reaction to Stimuli. 



Hydras were tested to find out their reaction to mechanical 

 and chemical stimuli. 



Mechanical Stimulation was carried out with a glass 

 rod drawn to a fine point. 



Slight stimulation of a tentacle leads to contraction of that 

 tentacle only. The tentacle often adheres to the rod for a few 

 seconds. 



Strong stimulation of a tentacle leads to its contraction over 

 the mouth, when the other tentacles bend up till they meet 

 and the head turns to one side, exactly as when catching prey 

 (capture response). This response is sometimes obtained by 

 rubbing on the insides of the tentacle bases, and sometimes by 

 touching the oral cone. Strong stimulation of the oral cone 

 leads to contraction. 



Stimulation of the outsides of the tentacle bases and of the 

 body in that region sometimes leads to contraction, but often 

 has no result. 



Gentle stimulation of the body generally has no result, but 

 sometimes leads to contraction, as strong stimulation always 

 does. 



Stimulation of the foot always leads to contraction of the 

 body (not of the tentacles, unless strong). This is probably 

 an adaptive reaction to movements of the object to which the 

 Hydra is attached. When contracted, the resistance to water 

 will diminish, and the animal will be less liable to be torn off. 



Gentle touches repeated at intervals of five seconds or two 

 and a half seconds sometimes have no effect even when carried 

 on for several minutes. More often they lead, first, to a 

 swinging away of the body, and finally to contraction after 

 a few minutes. In swinging away the body is bent just above 

 the foot region. As the body remains quite straight the 

 action must be confined to a small number of muscle-fibres 

 in the region of the bend, which is usually in a different part 

 of the body from the stimulation. This shows the existence 

 of a conducting mechanism. 



Chemical Stimulation. — When food, such as a piece 



