The Nerve Net of the Actinozoa

Abstract

1. Stimulation of the column of the anemone Calliatis parasitica elicits a graded series of responses. Each response is primarily due to the action of a single set of muscles. The means by which this effect is produced is physiologically simple. Part of the nerve net beneath the intact column behaves as a single conducting unit and the various muscles communicate with it. The muscles, however, are only activated through the facilitation of a series of impulses. Each muscle has its own appropriate frequency range, to which a facilitated response can be obtained. The range of frequencies is extraordinarily low, from 1 impulse in 1 sec. to 1 impulse in 10 sec. 2. "Interneural" facilitation can be demonstrated between adjacent conducting units of the nerve net of the disc. From various points on the disc several stimuli may be required to establish a conducting path to the sphincter muscle. The apparent continuity of conduction throughout the column in the intact animal is due to specialised tracts running vertically up the mesenteries. These are joined by a conducting ring in the neighbourhood of the sphincter. In addition to this "through-conduction" system, there is a general nerve net within the column wall in which interneural facilitation is evident. 3. The velocity of conduction in the general nerve net of the column has the slow rate of 10-20 cm. per sec. But for the sphincter-mesenteric system the rate is over 1 metre per sec. 4. The relative development of both interneural facilitation and of conduction velocity within the nerve net give consistent pictures of its physiological organisation. These agree with its morphological organisation. 5. Comparison with other species shows that the through-conduction system is a specialisation, developed in varying degrees in different species. It is most highly developed where protective reactions are most perfect, as in Calliactis. 6. The importance of the "impulse group" propagated from a stimulated sense organ as the natural unit in behaviour of Calliactis is discussed.