1 82 The Animal Mind 



49. Orientation to Gravity: Codenterates 



Among the ccelenterates, geotropisin is shown by certain 

 hydroids, whose stems have a tendency to curve upward 

 and their "roots" a tendency to grow vertically downward 

 when the animals are placed in a horizontal position (714). 

 The sea-anemone Cerianthus, whose normal position is head 

 upward, will right itself if placed in any other position, 

 though the righting reaction may be inhibited by contact 

 stimulation on the side of the animal. It ordinarily lives 

 with the body enclosed in a tube, and when taken from its 

 proper habitat it seems to "prefer" a position, even hori- 

 zontal, where the sides of the body are in contact with a 

 solid, to a vertical position with its sides uncovered (424). 

 The righting reaction of Hydra is not determined by gravity 

 at all ; the animal will take any position, vertical or hori- 

 zontal, but "seeks" always to have its foot in contact with 

 a solid (751 a). Some actinians have shown an interesting 

 modification of gravity reaction through what we may call 

 habit. Six specimens of Actinia equina were selected that 

 had been fixed to the rocks in an "upside-down" position, 

 that is, with the mouth end downward ; and six others that 

 had been right side up. In the first experiment all were 

 placed upside down ; the tendency to right themselves was 

 decidedly stronger in those which had been previously erect. 

 Similarly, when twelve selected in the same way were all 

 placed right side up, the ones that had previously been in 

 the reversed position showed a certain inclination to 

 reassume it (258). On the other hand, the orientation of 

 the polyp Corymorpha palma to gravity was entirely un- 

 affected by keeping the animal for a long time in a position 

 where it could not right itself; it assumed the upright 

 position as soon as it was set free (714). 



