Experimental studies of plankton luminosity have been made with specially designed photomultiplier equipment; this records (on a Servograph Recorder) the flashes given off either by a sample of plankton or by individual animals placed in a container in a dark observation chamber. The apparatus is described.

Apart from preliminary laboratory records on the flashing of Beroë, Pleurobrachia and Pyrosoma, and record of the general luminosity in the sea at a depth of 62 m (made with an underwater version of the apparatus), the main work has been a laboratory study of the luminescence of dinoflagellates and of the euphausiacean Meganyctiphanes norvegica.

The dinoflagellate experiments showed the inhibiting effect of light, both natural and artificial, on luminescence, and the stimulating effect of mechanical disturbance which may override the former inhibition. A rise in temperature increased luminosity and cooling reduced it. A difference in the pattern of the luminosity of plankton samples containing dinoflagellates was shown by the introduction of actively swimming animals of different kinds. The dinoflagellates in the samples were mainly Peridinium depressum, Ceratium horridum and C. tripos var. balticum.

The Meganyctiphanes experiments showed a spontaneous flashing in seventeen out of twenty-seven tests and in one test the animals maintained such activity for the greater part of a 24 h period. In addition to their bright flashing, the photophores would at times keep up a dull glow for long periods (especially see the observations of F. Körte in the Appendix). Spontaneous flashing does not only occur in the breeding season as had been suggested by Mauchline (1960).