The behaviour, histology, and ciliary and luminescent responses of Pyrosoma tetrazooids are described. Each zooid contains paired luminous organs made up of large cells packed with organelles that are concluded to be intracellular luminescent bacteria. Light emission can be evoked by photic or mechanical stimuli, and is always preceded (with a latency of at least 600 ms) by arrests of the ciliary cells of the gill bars, and concomitant ciliary arrest potentials. The gill bars are innervated by axons running under the ciliary cells; the luminous organs are not innervated. Ciliary arrests are evoked by photic stimuli and this appears to be the basis for transmission of light emission between zooids.