Excitability of nerve-free hydra

Abstract

THE simplest nervous systems known are those of Coelenterate polyps, of which hydra is one. Nerve cells of the common freshwater hydra occur in a diffuse, two-dimensional nerve net dispersed among the epithelial cells of the outer, ectodermal layer^1–4. There are also a few nerve cells in the inner, endodermal layer but these are scattered and do not seem to form a continuous net⁵. Nowhere in a hydra are there nerve cell concentrations or clusters of sufficient complexity to warrant being called ganglia. There have been many behavioural and electrophysiological investigations of hydra^6–9, but the function of the nervous system in the control of behaviour is still unclear. In other coelenterates epithelial cells have been shown to be capable of propagating behavioural signals^10,11. Several conducting systems coupled with spontaneously active pacemakers have been identified in hydra, but it is not known which of these conducting systems, if any, are neuronal and which result from activity in non-neuronal, epithelial cells. We have investigated the behaviour of Hydra attenuata in effectively nerve-free animals and show that such behaviour as remains, is controlled by non-neuronal cells.