1. The Outer Skin Pulse (OSP) system of the stolon of Salpa fusiformis was studied histologically and electrophysiologically.

2. The cells of the conducting epithelium are cuboidal, 4–10 μm in diameter and are connected by gap and tight junctions (Fig. 1). They have resting potentials of - 75 to - 96 mV.

3. Outer Skin Pulses are conducted as overshooting action potentials 84–104 mV in amplitude which are characterized by the absence of a hyperpolarizing undershoot during the repolarizing phase (Fig. 2 A). When OSPs are evoked at frequencies in excess of 2 s−1 a pronounced plateau appears in the repolarizing phase (Fig. 2C).

4. Tetrodotoxin blocks OSPs in a manner which suggests that a sodium current is responsible for activation of the action potential.

5. Addition of 15 mM manganese to the bath blocks OSPs. This effect was initially accompanied by a reduction in OSP amplitude and an increase in duration (Fig. 5). This observation was interpreted as indicating the presence of a calcium influx during the action potential.

6. The results are compared with those obtained from epithelial conduction systems in other organisms, other embryonic tissues and excitable cells in other non-vertebrate chordates.

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