The Nerve Net of the Actinozoa

Abstract

1. The relation of the "staircase" effects in the vertebrate heart, in the bell of Medusae and in the sphincter of Calliactis is discussed. Whereas in the heart the effect is due to an intrinsic increase of contractility of the whole contractile tissue, evidence is brought to show that the staircase in Calliactis and in Medusae depends upon neuromuscular facilitation by which each stimulus in a series reaches more and more individual muscle fibres. 2. The relation of different types of contraction in coelenterate muscle is discussed. It is shown how this depends upon the relation of the stimulation interval necessary for a facilitated response and the duration of a complete contraction and relaxation of the muscle. If these are about equal the step-like contraction of the sphincter of Calliactis results; if the facilitation interval is long a staircase similar to that of a Medusa bell is produced; if the duration of the twitch is relatively long a smooth contraction as in the mesenteric muscle of Calliactis is produced.