1. A method of administering measured local mechanical stimuli is described. Experiments were done upon the anemone Calliactis parasitica. 2. Mechanical stimuli show rapid apparent adaptation--partly due to simple mechanical causes such as contracture and passive deformation of the tissues. 3. When conditions are standardized a mechanical stimulus of sufficient intensity on the column gives a nervous impulse. These mechanical stimuli can be used in the same way as electric shocks to give facilitated responses. 4. Increasing the mechanical intensity of the stimulus shows a threshold below which no impulse is generated; and with further increase of strength, trains of increasing numbers of impulses. There exists a system capable of graded mechanical excitation. Gradation is also to be observed below the threshold since there can be summation of subliminal stimuli. 5. The excitable system appears to be oriented tangentially and responds to stretch rather than to pressure. 6. The excitable system is purely endodermal. The ectoderm and mesogloea act only as an integument. 7. On histological grounds there are grave difficulties in supposing that impulses set arise simply and directly in the numerous simple sense organs. The possibility is noted that the graded excitation of the mechanically sensitive system causes the nerve-net to fire off impulses. 8. The sensitivity of different parts of the animal varies greatly. In Calliactis the oral disk is as least 4000 times as sensitive as the column. 9. The combination of a simple mechanically excitable system with the gross morphological features of the bodily organization permits purposive and varied responses. Thus strong stimulation of the column leads to the closure reflex, whilst weak stimulation of the disk by contact or by water movements leads to appropriate responses connected with feeding or rejection.