PRODUCTION OF LIGHT 411 



and at a depth of a thousand fathoms. All the species of 

 Myctophum may be regarded as pelagic and nocturnal fishes 

 visiting the surface at night, but other species of the family are 

 probably truly abyssal and never taken at the surface. 



The Myctophidae are small fishes not more than a few 

 inches in length with a rather blunt snout and large eyes ; the 

 pelvic fins, as in other open-bladdered fishes, are abdominal in 

 position ; there is one rather large dorsal fin in the middle of 

 the back, and behind this usually a rudimentary or " adipose " 

 fin ; the tail-fin is forked. The small pearly organs are 

 situated in a series on each side along the ventral edge, in 

 groups on the sides near the lateral line, on the mandible, and 

 on the operculum ; in addition to these there are in some 

 species larger organs called sternchasers, on the dorsal edge in 

 front of the tail and others in front of and below the eye. 



In earlier times light-producing organs both in fishes and in 

 invertebrates were supposed by some to be accessory eyes. 

 Although this view can no longer be maintained, the emission 

 of light by the living fishes has only rarely been observed. 

 Dr. Giinther was one of the first to see and describe light 

 produced by these organs in a living Myctophum ; during a 

 gale while he was in the Channel Islands he obtained a living 

 specimen which was cast ashore ; the light was seen to proceed 

 from the special organs ; it was irregularly intermittent, some- 

 times well defined like a round spark, sometimes more diffuse. 

 It did not extend to the tail region which was probably 

 already paralysed, and it ceased with the life of the fish. Mr. 

 Guppy, on board H.M.S. Lark, took two specimens of a species 

 of Myctophum near the Cape of Good Hope, one of which was 

 alive ; the dead specimen displayed no luminosity even when 

 irritated, but the one that was still alive showed faint but un- 

 doubted light in the pearly bodies of the pectoral region which 

 were larger than the others ; the production of light was not 

 affected by any irritation. 



There is no close connection between the possession of 

 luminous organs and the conditions of abyssal life : just as 

 many surface fishes possess these organs, so many abyssal fishes 

 are destitute of them, and so far as we know have no power of 

 emitting light. Allied to the Myctophidie, are numerous other 

 fishes formerly united with them in the same family, some of 



