BEHAVIOR OF CCELENTERATA 



205 



28' 



\ 



tors. If the high temperature still continues, it changes position again, 

 and thus continues till the high temperature ceases or the animal dies. 

 The behavior resembles essentially that of infusoria under similar con- 

 ditions. The reaction is very ineffective under the conditions shown in 

 Fig. 127, owing to the 

 slowness of the move- 

 ments of Hydra. Most 

 of the animals in the 

 heated region finally die. 

 But if the animals moved 

 rapidly and far at each 

 change of position, then 

 those that moved away 

 from the heated side 

 would escape, and those 

 that moved in the 

 wrong direction the first 

 time would, after one 

 or two changes of di- 

 rection, likewise get out 

 of the heated region. 

 The reaction would be 

 of precisely the same 

 character as that of the 



infusoria. But the FIG. 127. Diagram of the movements of a number of 



action SVStem of Hvdra Hydras when the trough containing them was heated at the end 



e * 'to the left. Each of the small diagrams represents the move- 



is evidently adapted ments of a single Hydra. The figures i, 2, 3, etc., show the 



onlv for meetinff successive different directions in which the Hydra extended 



while remaining attached. The cross ( X ) between two num- 



Changed Conditions OVer bers indicates that here the animal released its foothold and 

 a VCrv limited area SUCh move< ^ ' n l ^ e direction shown to a new point of attachment. 



After Mast (1903). 



as may be escaped by a 



slight, slow movement. When the changed conditions cover too large 

 an area, the Hydra can only "try" its usual reaction; if this fails, it 

 must die. 



A decrease of temperature does not cause Hydra to change position. 

 As the temperature becomes lower, the animal merely becomes more 

 sluggish, contracting more slowly and at longer intervals, till finally, 

 near the freezing point, movement almost ceases (Mast). 



As we have seen on page 194, an internal condition hunger - 

 may induce the same locomotor reactions as are produced by continued 

 external stimulation. This is a matter which we shall take up again in 

 the account of food reactions. 



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