VI PHOSPHORESCENT ORGANS 181 
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, which frequents depths of over 2000 
fathoms, has not only the simple organs described above, but also 
others differing from the former in having an external pigmentary 
sheath, which are scattered all over the body at intervals of 1 to 
3mm. 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 benoitii 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. 
