io8 



BEHAVIOR OF THE LOWER ORGANISMS 



regulatory (beneficial) ; they give this reaction whatever the nature of 

 the unfavorable stimulus, even under circumstances where it is not 

 regulatory. This is an illustration of a characteristic general trait of 

 behavior in lower animals ; their reactions are commonly not specific, 

 but general in character. As we shall see later, this contraction is not 

 the final recourse of the stimulated ccelenterate. If stimulation con- 

 tinues, the animal usually sets in operation other activities, which remove 

 it from the stimulating agent. 



4. Localized Reactions 



Hydra. — In Hydra, intense stimuli restricted to a small spot on 

 the body or a tentacle usually produce contraction at that point, some- 

 times spreading much or 

 little, sometimes not at all. 

 This reaction is produced by 

 many sorts of stimuli. If 

 the contraction remains pre- 

 cisely localized, as it some- 

 times does, the body or 

 tentacle bends sharply at 

 the point stimulated. 



A precisely localized 

 chemical stimulus is pro- 

 duced in the following way. 

 . , . , , 7 , A fine capillary glass rod is 



Fig. 124. — A chemical (ch.) is brought against a 1 • • 



certain spot on one side of a Hydra (a). Thereupon dampened and its tip IS 



this spot contracts, bending the Hydra toward the side dipped in SOme powdered 



stimulated (b). 1 1 . _ F 



chemical. Methylene blue 

 or methyl green is convenient to use, since the distribution of 4he chemi- 

 cal in the water is easily seen by means of the color. The point of this 

 fine rod, covered with the chemical, is brought close to the body of a 

 Hydra. The chemical diffuses and reaches a small area on the body. 

 Local stimulation by heat may be produced with the simple apparatus 

 devised by Mast (1903). A glass tube is drawn out at its middle to 

 capillary size, then bent so as to form a loop. The two ends are 

 passed through a cork for support, and to them are attached rubber 

 tubes. In this way water of any desired temperature may be passed 

 through the fine tube, and this may be brought close against the body of 

 the animal at any desired point. 



When the strong chemical or the heat reaches a certain spot on the 

 body, this spot at once contracts, so that the body makes a knee-shaped 



