1. The spontaneous behaviour of the hydroid Tubularia results from the activity of a number of pacemaker systems in the polyp, there being a major pacemaker system in the distal stalk (the NP system), another in the hydranth body (the HP system) and individual pacemakers in each of the tentacles. The NP system produces single electrical pulses and bursts of three or more pulses, the HP system itself produces single pulses and short bursts of 2-4 pulses, and the tentacle pacemakers fire irregularly.

2. The HP and NP systems are mutually interactive. When the NP system fires in a burst it usually drives the HP system to fire concurrently. Single firing by the NP system sometimes triggers the HP system to fire, and single HP firing can trigger the NP system. Single HP firing is most likely to trigger the NP system if it comes long after the last NP firing, i.e. late in the NP spontaneous cycle. When the HP system independently fires a burst or is driven by the NP system to fire a burst, it drives pacemaker systems in the proximal tentacles to fire concurrently.

3. Firing of the HP system is associated with a behavioural response termed a concert. During a concert the proximal and sometimes the distal tentacles are synchronously elevated. During the longer NP-HP bursts the neck contracts. A peristaltic wave sweeps down the proboscis following the tentacle elevation. Concerts without neck contraction result in mixing of fluid in the enteron chamber of the proboscis; concerts with neck contraction result in tidal exchange of fluid between the neck chamber and the proboscis chamber.

4. Stimulation of the conducting system in the stalk termed the triggering system (TS) can trigger the NP system to fire. A stalk stimulus is most likely to trigger the NP system if it comes within a few seconds of NP firing, i.e. early in the NP spontaneous cycle.

5. Firing of the NP system of one polyp in a colony can trigger the NP systems of connected polyps. Interpolyp triggering is probably mediated through the TS conducting system. The coupling between NP systems of connected polyps results in synchronization of NP bursts throughout a colony. Because of the coupling between NP and HP systems within single polyps the longer HP bursts, which are those occurring when the HP system is driven by the NP system, are also usually synchronous in connected polyps. This results in concerts with neck contraction being nearly synchronous throughout a colony. Such synchronization facilitates transfer of fluid between the two enteron chambers of polyps during concerts.

6. Prolonged, low-frequency stalk stimulation inhibits spontaneous activity of a polyp, the inhibition increasing with increasing stimulus frequency.

7. The large size of the spontaneous electrical pulses suggests that they are not the result of nervous activity, but possibly due to activity of more primitive co-ordinating mechanisms.

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