BEHAVIOR OF CCELENTERATA 197 



Other favorable ones are present. The behavior represents a compro- 

 mise of the various needs imposed upon the animal by its physiological 

 processes. 



In the sea anemone Antholoha reticulata, according to Burger (1903), 

 the requirements for retaining a given position are extraordinary. This 

 animal is usually found attached to the backs of crabs ; it is thus carried 

 about, and finds much opportunity for obtaining nourishment. If re- 

 moved from the crab's back, the animals attach themselves to the stony 

 bottom and spread the tentacles. But after four or five days they re- 

 lease their hold on the bottom and invert themselves, directing the foot 

 upward. Now when a crab's hmb comes in contact with the foot, the 

 latter attaches itself and folds about the limb, so that the anemone is 

 dragged about by the crab. It now, in the course of several hours,, 

 climbs up the crab's leg to its back, where it establishes itself. The sea 

 anemone thus by its own activity attains the extraordinary situation 

 where it is usually found. The whole train of action is like that shown 

 in the complicated and adaptive instincts of higher animals. 



3. General Reaction to Intense Stimuli 



The most characteristic reaction of the coelenterates to intense stimuli 

 of all sorts is a contraction of the whole body. In Hydra and the sea 

 anemones the body is thus shortened and thickened, becoming more 

 nearly spherical. The animals thus shrink close to the substratum 

 and present less surface than before to the stimulating agent. In the 

 medusae the sudden contraction of course carries the animal away from 

 the stimulating object. The first contraction is usually repeated many 

 times, thus inaugurating a period of swimming by which the animal may 

 be widely removed from the stimulus. Such contractions occur in re- 

 sponse both to general stimulation and to local stimulation, if the latter 

 is very intense. 



Under most circumstances the contraction of Hydra or the sea anem- 

 one of course tends to remove the organism from any source of danger, 

 rendering it for example less likely to be seized by a predatory animal. 

 But the reaction takes place in the same way under circumstances in 

 which it is of no defensive value. If the foot of the attached Hydra 

 is strongly stimulated, the animal contracts as usual ; the contraction is 

 then of course toward the source of stimulation, not away from it. If 

 the entire vessel containing the animals is heated to 30 degrees, the 

 Hydras contract, though this of course does not tend to remove them 

 from the high temperature. It is clear that for all sorts of stimuli that 

 are unfavorable these animals have a certain reaction which is usually 



