214 FOUNDERS OF OCEANOGRAPHY 



finally death, and that the infection could be transmitted 

 from one sand-hopper to another. (Plate XIII, Fig. 1.) 



It is evident, then, that the luminescence of a larger 

 marine animal is not necessarily due to the production of 

 light from its own body, but may be caused by an invasion 

 of photobacteria. 



Protophyta. — Minute unicellular plants in the surface 

 layers of the sea are probably the cause of a good deal 

 of the duU, generally diffused glow, which has been called 

 " milky sea " in the Far East, " white water " in the Gulf of 

 Aden and elsewhere. Sir John Murray considered that 

 the unicellular plant Pyrocystis (possibly a Dinoflagellate 

 allied to Noctiluca) is the chief cause of the diffused Hght 

 often seen in tropical seas in calm weather. 



Protozoa. — Many of the Flagellata exhibit luminescence, 

 especially those belonging to the group Dinoflagellata (such 

 as Ceratium Sbiid Peridinium), which have been known to be 

 luminous since the time of Ehrenberg (1831), and possibly 

 earher, I have proved to my own satisfaction, through the 

 microscope, that the bright sparkles in a sample taken from 

 a luminescent sea on the west coast of Scotland were caused 

 by the abundant Dinoflagellate Ceratium tripos (Plate XIV, 

 Fig. 2) ; and similarly in the Southern Ocean, off the 

 Cape of Good Hope, I once found that the organism lighting 

 up the sea by night and colouring it almost blood-red by 

 day was a small red Peridinium present in extraordinary 

 abundance. 



The aberrant Dinoflagellate Noctiluca scintillans (Plate 

 XIV, Fig. 1) is the generally recognized cause of a great deal 

 of the silvery luminescence of our home seas round the coasts 

 of North-west Europe in summer and autumn, when this Uttle 

 organism is sometimes so abundant that every dip of a 

 cup in the sea will contain hundreds, and every tide leaves 

 pink-coloured masses of their bodies piled up on the sands. 

 In the Irish Sea, for example, Noctiluca is very generally 

 present in the plankton, and enormous swarms appear from 



