BEHAVIOR OF CCELENTERATA 



193 



to some other simple stimulus. It will be found instructive to examine 

 the different conditions on which depends whether the animal shall or 

 shall not retain a given position in which it finds itself. It will be found 

 that the matter is not an entirely 

 simple one. 



Let us take first the case of Hydra. 

 Suppose the animal to be placed on a 

 horizontal surface with head down- 

 ward and foot upward. It does not 

 retain this position, but bends the 

 body, placing the foot against the 

 bottom, releases its head, and 

 straightens upward. This is what 

 is commonly called the "righting" 

 reaction. In Hydra it is not due to 

 a tendency to keep the body in a cer- 

 tain position with reference to gravity, 

 for the animal may remain attached 

 to the bottom, with head projecting 

 upward, or to the surface film, with 

 head projecting downward, or to a 

 perpendicular surface, with the body 

 transverse or oblique to the direction 

 of gravity. There is even apparently 

 a certain tendency to direct the head 

 downward. Thus out of 100 green 

 Hydras attached to a perpendicular 

 surface, 96 had the head lower than 

 the foot, 3 were horizontal, and 1 had 

 the head directed upward. It is thus 

 clear that the righting reaction of a 

 Hydra which has been inverted on 

 the bottom cannot be due to any 

 unusual relation to the direction of 

 gravity. 



To what, then, is the reaction 

 due ? Evidently there is a tendency to 

 keep the foot in contact with a surface, for the body is bent till the foot 

 comes in contact. But this is not all ; the reaction does not stop at this 

 point. There is likewise a tendency to keep the head free, for it is re- 

 leased. But still this is not all, for now the body is straightened ; then 

 the tentacles are spread out symmetrically in various directions. 



