1. In the apparent absence of any immediate external stimulation the retractor muscles of Cucumaria show slow contractions which take 1-2 min to develop fully. These contractions develop the maximal possible tension and are of an all-or-nothing character.
2. The contractions are neurogenic in origin, ceasing if the retractor motor complex is destroyed and being only very rarely shown by a preparation in which the motor complex is de-afferented.
3. The slow contractions may be partially or fully inhibited by stretching the longitudinal muscle.
4. Slow contractions may be released by electrical stimulation of the radial nerve. The threshold of stimulation is higher than that for the previously described ‘quick’ and ‘delayed’ responses.
5. The slow contractions released by electrical stimulation are normally maximal, but partial slow contractions may be released in certain conditions. Slow contractions are not more frequently released by repetitive stimulation of the radial nerve.
6. The occurrence of a slow contraction modifies the characteristics of the quick and delayed responses.
7. When the activity of all five retractor muscles is recorded, co-ordinated slow responses may be observed; in such responses excitation appears to spread from one radius and to be conducted independently to the other four.