INTRODUCTION

Both the inner and outer epithelia of salps propagate action potentials (Mackie & Bone, 1977). Skin pulses in the outer epithelium underlying the test (OSPs) evoked by mechanical or electrical stimulation of the epithelium ‘enter’ the brain and may alter the regular rhythmic locomotor activity (Mackie & Bone, 1977; Anderson et al. 1979). The route of'entry’ has not been determined, but has been assumed to be via the axons of the scattered mechanoreceptor sensory cells lying in the outer epithelium. The OSP system would thus operate to extend the sensory field of such cells, as in the appendicularian Oikopleura (Bone & Mackie, 1975; Bone & Ryan, 1979) where two sensory cells are coupled to a conducting epithelium.

Salps alternate generations between the solitary asexual oozooid, and the aggregated sexual blastozooids (budded from the stolon of the oozooid). The linked blastozooids form chains, along which OSPs pass to regulate the locomotor behaviour of individual zooids in the chain. The zooids are not linked by gap junctions, and OSPs pass along the chain in a complex.manner, involving alternating epithelioneural and neuroepithelial synapses (Bone, Anderson & Pulsford, 1980; Anderson & Bone, 1980). The OSP system of the oozooid generation is less well understood, although it is known that OSPs in the outer epithelium of the oozooid propagate into the stolon, where they have been studied by Anderson (1979). This paper shows that oozooids possess a similar system of neuroepithelial synapses to that of blastozooids, and that these ‘ drive’ OSPs in the same way as occurs during the regenerative transmission of OSPs along the blastozooid chain.