PELAGIC FLOATING CRUSTACEA 151 



special glands on the surface of the body, a sub- 

 stance 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 Halocypridae 

 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. 



There are certain peculiarities in the structure of 

 the eyes in some plankton Crustacea which suggest 

 that the sense of sight is of special importance to 



