On the Functions and Mode of Action of the Nematocysts of Hydra.

Summary. (1) The threshold for discharge in response to mechanical stimulation of the stenoteles and desmonemes of Hydra is lowered by food extracts, in the same way as has been shown by Pantin to occur in sea anemones. (2) The tentacles are attached during locomotion by discharge of the atrichous isorhizas. Food extracts have the effect of inhibiting the discharge of this type of nematocyst. The duration of mechanical stimulation necessary to bring about discharge is greater for atrichous isorhizas than for stenoteles. (3) The holotrichous isorhizas have only been found to be discharged when stimulated by animals which are not normally eaten by Hydra and evoke no feeding reaction. Their threshold is unaffected by food extracts. Their function is concluded to be defensive. (4) Normal feeding and locomotion in Hydra are described, and the relation of the reactions of the nematocysts to chemical and mechanical stimuli in their normal functions is discussed. My thanks are due to Professor H. Munro Fox, F.R.S. in whose laboratory this work was carried out, to Dr. C. F. A. Pantin, F.R.S., for helpful discussions and advice, and to Messrs. George Newnes Ltd., for permission to reproduce fig. 1.