The striking phenomenon known as the "phosphorescence of the sea" is familiar to every ocean voyager, and is seen from time to time on our own coast. On a dark night the crest of every wave often seems to break in a pale glow, the wake of the vessel is a trail of light, and an oar dipped in the water seems on fire. This luminosity is due to the animals of the plankton, largely to the lowly Protozoa and the jellyfishes, but in part also to certain Crustacea. A number of pelagic Copepoda have been shown by Giesbrecht to secrete, from special glands on the surface of the body, a substance which becomes luminous on coming in contact with the water. Even specimens which had been dried were found to give out light on being wetted. Some pelagic Ostracods of the family Halocypridæ have been observed to emit clouds of a luminous secretion from a gland in the neighbourhood of the mouth. A similar habit has been seen, as already mentioned, in certain deep-sea Prawns and Mysidacea, which may perhaps belong to the deeper part of the mesoplankton rather than to the bottom fauna. The complex light-producing organs of the Euphausiacea have already been described in dealing with deep-sea Crustacea. A great many species of this group, however, are members of the epiplankton, and in these the phosphorescent apparatus is quite as fully developed as in species coming from greater depths. _Meganyctiphanes norvegica_ (Fig. 24, p. 56), which is one of the largest of the Euphausiacea, is common at no great depths in many places in British seas. If a jar of sea-water in which specimens of this species are swimming be brought into a dark room, a tap on the glass will cause the photophores to flash out like a row of tiny lamps along the side of the body. After shining for a few seconds the light dies out, to appear again if the tapping be repeated.