208 



BEHAVIOR OF THE LOWER ORGANISMS 



Sometimes a slight reaction of a different character supervenes after 

 the stimulus has been repeated many times. The animal begins to 

 shrink slowly away from the region where the drops are falling, so that 

 in the course of time the disk has been withdrawn much farther be- 

 low the surface, though no decided reaction has occurred to any one 

 stimulus. 



8. REACTIONS TO CERTAIN CLASSES OF STIMULI 



In the foregoing sections we have taken up reactions to mechanical 

 stimuli, heat and cold, and chemicals; we shall have occasion to con- 

 sider some of these further in the account of food reactions. There are 

 certain other classes of external stimuli which may play a part in deter- 

 mining behavior in these animals ; these we will take up separately. 



A. Reactions to Electricity 



Induction shocks have been much employed in experimental work 

 on contraction in ccelenterates. The results of such stimulation do not 



I 2 3 5 



FIG. 128. Reaction of an attached Hydra to a constant electric current of moderate inten- 

 sity. 1-5, successive stages in the reaction. After Pearl (1901). 



differ greatly from those produced by other forms of stimulation (me- 

 chanical, etc.), local or general contractions occurring in dependence 

 on the strength of the current. These may be followed by locomotor 

 movements. 



The effects of the constant electric current are more peculiar and of 

 greater interest. They have been studied in Hydra by Pearl (1901); 

 in the medusa Polyorchis penicillata by Bancroft (1904). 



Hydra. In Hydra the constant current causes local bendings of 

 the body similar to those produced by sharply localized chemical and 

 thermal stimuli. If a weak current is passed through the water trans- 

 versely to the Hydra, the animal contracts on the anode side, at a point 

 a little above the foot, thus bending the body (Fig. 128). At the same 

 time or a little before, the tentacles which were in line with the current 

 contract (Fig. 128, a). Sometimes, further, there is a contraction on 



