VI PHOSPHORESCENT ORGANS I 8 I 



of the lens-like body, somewhat after the fashion of an iris- 

 diaphragm. The organ is supplied by a branch of the fifth 

 cranial nerve. Between such simple and complex organs as 

 those above described there are various other types which are 

 more or less intermediate in character. 



A particular type of phosphorescent organ is not necessarily 

 restricted to the same species ; both the simplest and one or more 

 of the more complex types may be represented in the same Fish. 

 Thus, Opostomias micripnus,^h\ch. frequents depths of over 2000 

 fathoms, has not only the simple organs described above, but also 

 others diifering from the former in having an external pigmentary 

 sheath, which are scattered all over the body at intervals of 1 to 

 3 mm. There are also larger and still more complex organs which 

 are disposed in two parallel rows along each side of the body ; 

 and finally, the same species has special luminous organs on a 

 median chin-barbel, and also on an elongated fin-ray pertaining 

 to the pectoral fin. 



The light emitted by phosphorescent organs is probably of use 

 to deep-sea Fishes in enabling them to seek and detect their 

 prey in the sunless depths which they frequent. The position 

 of the organs on the sides and ventral surface of the body, and 

 the frequent presence of special luminous organs in the vicinity 

 of the mouth, render them admirably adapted to light up the 

 water in front of and beneath the Fish, while the existence of 

 optical accessories for intensifying the luminous beams, and for. 

 regulating their distribution, combined with an abundant nervous 

 supply, suggests that the emission of light is under the control of 

 the Fish, and may be varied as the occasion requires. That 

 these organs may also be defensive, in some instances at all 

 events, seems not improbable. A flash-light from the dorsal 

 luminous organ or " stern-chaser " of Scopelus ienoitii would 

 probably dazzle and frighten an enemy in hot pursuit of the 

 Scopelus. The use of phosphorescent organs as baits or lures 

 for enticing prey has already been alluded to. There is some 

 evidence that the colour of the emitted light differs in different 

 Fishes ; and as there is considerable variety in the precise dis- 

 position of the organs, it seems probable that in deep-sea Fishes 

 recognition lights may take the place of the recognition colours 

 and sounds of those whose lot is cast in a sunnier habitat. 



