1. Conduction of excitation in response to local mechanical or electrical stimulation has been studied in various hydroid species.
2. There are systems in the coenosarc of the stems and stolons of all species which conduct excitation at rates of from 1 to 3.5 cm./sec.
3. Two physiological types of conducting systems have been found.
(a) Through-conducting systems, showing: all-or-none response, sharp threshold, reproducibility.
(b) Local systems, showing: spread of response dependent upon stimulus strength, no sharp threshold, responses which decline with increasing distance from the stimulated point, variability.
4. Colonial co-ordination is better developed in those species whose colonies are structurally better developed. It is effected in most species by either (a) or (b). Both types are present together in Hydractinia echinata.