1. Certain features have been supposed to characterise the nerve net. Response is said to vary with the strength of stimulus: and while conduction may occasionally take place in an all or nothing manner, yet in general conduction is supposed to take place with a decrement. To investigate these points, the responses of Calliactis parasitica to mechanical and electrical stimuli have been investigated.
2. Electrical excitation of the column of the anemone shows that a response results from a succession of stimuli and not from a single stimulus. The character of the response is independent of the strength of the individual stimuli. It depends solely upon the number of stimuli and upon the interval of time between them. All responses are highly developed facilitation phenomena. Each electrical stimulus induces a single excitation impulse in the nerve net.
3. But a mechanical stimulus is followed by the discharge of a battery of impulses from the sense organs. These increase in number and frequency with the intensity of the mechanical stimulus. A response may therefore vary with the strength of a mechanical stimulus, but only in relation to the number and frequency of impulses discharged by the sense-organs.
4. Stimulation of the intact column of the anemone shows complete conduction over its whole nerve net. Mechanical stimulation of the disc appears to show conduction with a decrement. Such a stimulus excites a battery of impulses. Each impulse is conducted without decrement, but it facilitates the entrance of succeeding impulses into adjoining sections of the disc nerve net.
There is no decrement of excitation strength under any conditions in the nerve net. But there may be a numerical decrement as a battery of impulses spreads from a stimulus.
5. The nerve net is physiologically similar to ordinary nerve. A method is described for determining the strength duration relationship for the threshold of electrical excitation of the nerve net. The relation is of the usual form with a chronaxie of about 2-4σ. The nerve net has well-defined relative and absolute refractory periods: the latter is about 40-65σ. The relation of facilitation to refractory period is considered.
6. The whole nerve net of the column of the anemone acts as a conducting layer in its most simple form, directly transmitting excitation from the stimulus to the muscle. Facilitation takes place between the nerve net and its appropriate muscles. Conduction in the disc nerve net involves facilitation between parts of the nerve net in addition.
7. The true characteristics of the nerve net are diffuse conduction and the extreme development of facilitation. Diffuse conduction may be total, as in the column, or restricted, as in the disc of Calliactis. Facilitation may be between the nerve net and the muscles, or between parts of the nerve net.