VENOM, ELECTRICITY, LIGHT, AND SOUND 153 



is due to the luminous nature of the slime secreted by the 

 gland-cells. These manufacture a substance containing phos- 

 phorus, and this is oxidised by the oxygen supplied to the cells 

 by the blood to produce the gleam. To put the matter into 

 the language of biochemistry, a substance called luciferin, 

 secreted by the gland-cells, is burnt to oxyluciferin in the presence 

 of a ferment known as luciferase. In the vast majority of luminous 

 fishes the substance is burnt within the cells in which it is 

 manufactured, but a fish has recently been studied in which the 

 cells secrete the material for oxidation to the exterior and it is 

 burnt outside the cell. This fish (Malacocephalus) , a member 

 of the tribe of Grenadiers or Rat-tails (Macruridae) , is taken in 

 large numbers along the outer edge of the continental shelf 

 from Ireland to Morocco in depths of one hundred and fifty 

 fathoms or more, and possesses a remarkable luminous gland 

 lying between and behind the pelvic fins (fig. 62A). 



The purpose of the light organs of oceanic fishes is largely a 

 matter for conjecture. The use of the luminous bulbs and 

 barbels of the oceanic Wide-mouths (Stomiatoidea) and Anglers 

 (Ceratoidea) as lures has already been described. In other fishes 

 the emission of light is almost certainly defensive rather than 

 oflfensive. In the Grenadier just mentioned, for example, it 

 seems probable that a sudden beam of light emitted from the 

 gland between the pelvic fins would tend to confuse an enemy 

 and cover the retreat of the pursued in the same way as does 

 the ink-cloud of the Cuttle-fish. The "stern-chasers" of the 

 Lantern-fishes {Myctophidae) may serve a similar purpose, a 

 sudden flash from the tail being used to dazzle or even frighten 

 the pursuer. To explain the photophores and other organs 

 on the head and body is rather more diflficult. It is generally 

 assumed that they enable fishes living in the depths of the 

 ocean — the region of eternal night, as one author describes it — to 

 seek for and detect their prey. This may be true in part, but 

 it must be remembered that the same light which illuminates 

 the prey renders its owner equally conspicuous and liable to 

 be hoist by his own petard! Further, an extensive study of 

 the fishes inhabiting the oceans shows that there is no certain 

 connection between the possession of light organs and a life 

 in the abyssal depths. Many fishes spending the greater part 

 of their times at or near the surface have these organs well 

 developed, whilst a number of forms known to live permanently 

 at considerable depths are without them. 



In considering the function of luminous organs it is important 



