LUMINESCENCE IN THE SEA 213 



and lowly organized. Luminescence is produced also in 

 the case of a few land animals and plants, such as some 

 earth-worms, millipedes, and various insects (beetles), the 

 best known of which are glow-worms and fire -flies ; but is 

 not known to occur in any fresh-water organism. It is 

 therefore a widespread, but by no means universal, accom- 

 paniment of life — a vital phenomenon, only manifested 

 by certain living things, and by these only under certain 

 conditions. 



In the sea the organisms that give rise to luminescence 

 range from the simplest minute unicellular forms (Protozoa, 

 Protophyta, and Bacteria) up to Fishes, and the modes of 

 emittiQg the light and the appearances thus produced are 

 most varied. The following list is not intended to be 

 exhaustive, but merely to give a few examples of each of the 

 cliief kinds of organisms that contribute most notably to the 

 different appearances of luminescence : — 



Bacteria. — Many of these micro-organisms (e.g., the 

 various forms of Photohacterium and Microspira) cause a 

 flickering glow in the water, on wet sand, and on the bodies 

 of fishes and other larger organisms. Fishermen and 

 naturahsts since the days of Aristotle have noticed that 

 dead fish may glow in the dark, and this is not due to the 

 bacteria of putrefaction, but to the photobacteria of the 

 living fish, as when putrefaction sets in the luminescence 

 ceases. 



In other cases the photobacteria may invade the body 

 of a larger organism, give rise to a disease, and cause it to 

 glow in the dark. The late Professor Giard, while walking 

 (in 1889) on the sands of Wimereux at night, noticed spots 

 of light at his feet which moved from place to place, and, 

 on catching some of these, found them to be living, but 

 enfeebled, " sand-hoppers " (the Amphipods Talitrus and 

 Orchestia). Investigation in the laboratory showed that 

 the body was infested with photobacteria, that these caused 

 progressive enfeeblement of the muscular system, and 



