The Nerves and Muscles of Medusae

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Abstract
1. The responses of Geryonia proboscidalis to electrical and mechanical stimulation have been studied with particular attention to the transmission of excitation. 2. There are two kinds of excitation transmitted across the undersurface of the bell; first, the symmetrical beat is co-ordinated by an isotropic conducting system which acts in a through-conducting, all-or-nothing manner, and produces brief contractions of the circular muscle; secondly, the movements of the manubrium are co-ordinated by a radial conducting system, which produces a maintained contraction of the radial muscles of the manubrium. 3. The independence of these two conducting systems is shown by the simultaneous and yet distinct transmission of the two kinds of excitation in different directions. 4. The contraction wave is not transmitted through the ring nerves faster than over the rest of the bell. 5. The manubrium bends towards a stimulated tentacle, and all the tentacles contract simultaneously, co-ordinated by a through-conducting pathway in the marginal nerve. There is evidence that a polarized pathway carries excitation from this system to the rhythmical marginal centres. 6. Observations of Geryonia held with the axis horizontal show that the steering action is due to an asymmetrical maintained contraction of the velum. 7. The known histological details of the nervous system are compared with the physiological pathways demonstrated by experiment. The differentiation of a primitive isotropic nerve net into distinct tracts with separate functions is essential for the several rapid feeding and swimming activities.

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