1. The work described is a continuation of the work on the peristomial cilia of Stentor previously reported (Sleigh, 1956). The same methods were used in making the observations.
2. A cut across the peristome row of cilia did not affect the wave velocity, but the frequency distal to the cut was usually altered. In twenty-two cases the distal frequency was decreased, and in seven cases it was increased.
3. Cilia in the gullet region are smaller and closer together than those in the main part of the row; these cilia show the same frequency as the other cilia, but a smaller wavelength and wave velocity. The number of cilia in every metachronal wave of the row is the same, as is the number of cilia stimulated in unit time by a single metachronal impulse in any part of the row.
4. The wave velocity is dependent on the number of cilia through which the conduction is passed. The transmission is thought to be the result of a rapid conduction process between the cilium bases and a slower build-up of an excitatory state in the cilium.
5. The frequency of a group of cilia is dependent upon the activity of a pacemaker at the beginning of the group; this pacemaker is probably a cilium with a suitable rate of beat. Frequency is limited by the rate of stimulation or the rate of contraction, whichever is the slower.
6. It is suggested that there are some five excitation stages in the excitation of each cilium, and that each stage takes about 1 msec.; this is comparable with the delay at a nerve synapse.
7. The results of these experiments and observations on ciliary activity indicate that the pacemaker and transmission properties of ciliated tissues and cardiac muscle are comparable in many respects.