466 H. S. Jennings, 



position, but bends the body, placing the foot against the bottom, 

 releases its head, and straightens upward. Aiptasia shows the 

 same reaction. In neither of these animals is the reaction due to 

 a tendency to keep the body in a certain position with reference 

 to gravity, for both keep the body indifferently in any position 

 with reference to the pull of gravity, provided that the foot is 

 attached and the disk and tentacles can be spread freely. To 

 what then is the reaction due? Evidently there is a tendency to 

 keep the foot in contact with a surface, for the body of the inverted 

 Hydra is bent till the foot comes in contact. There is likewise a 

 tendency to keep the head free, for it is released. But this is not 

 all, for now the body is straightened, then the tentacles are spread 

 out symmetrically in all directions. It is clear that the reaction is 

 directed toward getting the organism into a position that may be 

 called "normal," and this normal position has various factors — 

 attachment of foot, freedom of head, comparative straightness of 

 body, and tentacles outspread. 



Suppose now that our Hydra has reached this position, and all 

 the conditions remain constant; is this sufficient.^ We find that it 

 is not. If the conditions remain so constant that no food is 

 obtained, the Hydra becomes restless and changes the position 

 of its body repeatedly, though still retaining its attachment by the 

 foot. Later even this is given up, and the animal, of its own 

 internal impulse, quite reverses the position attained through the 

 "righting reaction." It now bends the body, attaches the head, 

 and releases its foot, thus bringing it back into the inverted 

 position. 



Is this because the irritability of head and foot have become 

 reversed, so that the head now tends to remain attached, the foot 

 free.^ Apparently not, for no sooner has the animal taken the 

 inverted position than it draws its foot forward and now performs 

 the "righting reaction" again, so that it stands once more on its 

 foot. These alternations of behavior are repeated, and we find 

 that by this means the animal is moving from place to place (see 

 Wagner, 1905, Fig. 3). 



It seems clearly impossible to refer each of these acts or the 

 whole behavior to any particular present external stimulus. An 

 internal state — hunger — drives the Hydra to move to another 

 region, and these different opposite acts are the means by which 

 another region is reached. Each phase of the locomotion is 



