A Response to Gravity in Young Hydra

WHILE keeping cultures of Hydra vulgaris Pallas for class work it was noticed that whereas adult animals seldom changed their position, buds immediately after their separation from the parent rapidly made their way up the side of the tank to the surface of the water. This reaction seemed worthy of further investigation, since previous workers on the behaviour of Hydra have not made any distinction between adults and buds. Upward movement has generally been regarded as a response to lack of oxygen or to a gradient in oxygen concentration, rather than as a gravity reaction1,2,3. I have found that the upward migration of recently separated buds is a response to gravity and not to lack of oxygen, or to a gradient of oxygen concentration : It takes place both when the water is at air saturation and there is no gradient of oxygen concentration, and also when the oxygen concentration is arranged to be lowest at the top and highest at the bottom of the vessel containing the animals.