Motion and locomotion thumb|upright|Hydra attached to a substrate If Hydra are alarmed or attacked, the tentacles can be retracted to small buds, and the body column itself can be retracted to a small gelatinous sphere. Hydra generally react in the same way regardless of the direction of the stimulus, and this may be due to the simplicity of the nerve nets.
motion and locomotion thumb|upright|hydra attached to a substrate if hydra are alarmed or attacked the tentacles can be retracted to small buds and the body column itself can be retracted to a small gelatinous sphere hydra generally react in the same way regardless of the direction of the stimulus and this may be due to the simplicity of the nerve nets