PHOTOGENIC MARINE ANIMALS. 17 



Though it is many years (1840) since Dr. Bennett captured, 

 at the surface of the sea during a whaling voyage, a small Shark 

 which emitted spontaneously a general and vivid greenish lumi- 

 nosity — with the exception of a ring round the throat — as it swam 

 at night, and for some hours after death, it is only recently that 

 the labours of Johann* and Burckhardt have made us acquainted 

 with the remarkable prevalence of luminosity in the Selachians. 

 No less than eleven species, besides Bennett's form, have this 

 property. The distribution of the luminous organs varies in 

 almost every species ; in some occurring on the ventral surface, 

 in others along the trunk, on the bases of the fins, on the gill 

 region, and on the head. These organs consist of small epi- 

 dermal elevations, which present externally layers of epidermal 

 cells, some of the lower containing a prismatic corpuscle, pig- 

 ment, a basal strand of fibrous tissue, and a nerve. The whole 

 surface supplied with these structures is lit up in the dark. 

 Thus Beer, who examined Spinax, states that the entire ventral 

 surface from the snout to the root of the tail glowed with a 

 feebly shining greenish lustre, as if it had been impregnated 

 with phosphorus, or had been coated with luminous paint — with 

 this difference, however, that the luminosity appeared and dis- 

 appeared at short intervals, but invariably increased in intensity 

 before it vanished. It was vivid enough to enable him to see it 

 at a distance of from three to four metres, equalling three to four 

 yards (upwards). During life, therefore, the nervous system 

 appears to control the emissions of light ; and the glow after 

 death is probably due to the loss of the inhibitory power per- 

 mitting the continuous but final luminosity, arising from the 

 oxidation of the cell-contents. 



The varied distribution of these photogenic organs in both 

 Teleosteans and Selachians would not seem to point to any 

 definite purpose of allurement. The active habits of the Sharks, 

 besides, would negative such a proposition. Their pelagic life, 

 moreover, from the surface downwards, lends little support to 

 the view which would illuminate the abysses of the ocean by 

 such a provision in the fishes found only there. The splendour 

 of the spectral colours of such as Lcemargus rostratus during 



* Zeitsch. f. w. Zool., Ed. lxvi. 1899. 

 Zool. 4th ser. vol. X., January, 1906. c 



