Tne Behaviour of Hydra in Response to Gravity.



1. Buds of Hydra vulgaris immediately after separation from the parent migrate upwards towards the water surface. This reaction is a negative geotaxis, and not a response to a gradient in oxygen concentration; it takes place in water of any oxygen concéntration.

2. Hydra is known to be photopositive. The gravity reaction of buds is much more strongly affected by light coming from below than by light coming from the side. Animals which have been grown for two generations in bottom light behave in the same manner as normal animals from stock cultures. This effect of bottom light is discussed in terms of the known mechanism of the animal's light reaction, and it is concluded that no new complexity in type of response or in neuromuscular organization need be postulated to account for it.

3. An increase in carbon dioxide content of the water could not be shown to have any significant effect on the strength of the gravity reaction of buds, but evokes negative geotaxis in adult Hydra. Neither an equal lowering of pH by hydrochloric acid nor a reduction in the oxygen concentration has any affect.

4. The gravity response of buds lasts for at least three days after separation from the parent.

5. If a freshly separated bud is given food it ceases its upward migration.

6. Walking of buds is fastest at about 22°C. Abnormalities in locomotion begin to appear at slightly higher temperatures, and at 26°C. walking stops.

7. In upward migration the orientation to gravity is direct, without trial movements.

8. The gravity reactions described are of biological significance. That of buds acts as a method of distribution and prevents local overcrowding. The response evoked in adults by carbon dioxide serves to bring them up to the surface when there is likely to be a shortage of oxygen lower down.