302 FISHES. 



organs are considered by some naturalists true organs of 

 vision (accessory eyes), the function of the latter being left 

 unexplained by them. 



Although, thus, these organs morphologically differ from 

 each other, there is no doubt that the functions of all have 

 some relation to the peculiar conditions of light under which 

 the fishes provided with them live ; these fishes being either 

 deep-sea forms or nocturnal pelagic kinds. There are three 

 possible hypotheses as to the function of these organs : — 



1. All the different kinds of organs are sensory, or, in 

 other words, accessory eyes. 



2. Only the organs with a lenticular body are sensory, 

 and those with a glandular structure produce and emit 

 phosphorescent light. 



3. All are producers of light. 



There are very serious objections to adopting the first 

 view. Scopelus and Argyropclems possess not only perfectly 

 developed, but even large eyes, specially adapted for a 

 nocturnal life ; and therefore accessory organs of vision must 

 appear to be quite superfluous to them. On the other hand, 

 in Deep-sea fishes without external eyes, which would seem 

 to especially require these metameric organs of sense, they 

 are invariably absent. And, finally, it is quite inconceiv- 

 able that the glandular structures should have the faculty 

 of conveying impressions of light to the nervous centre. 

 The second supposition seems therefore to be nearer the 

 truth ; and is supported by the fact that the glandular organs 

 of Scopeli have actually been observed to gleam with phos- 

 phorescent light, and by the obvious morphological similarity 

 of the organs with a lenticular body and retina-like membrane 

 to an organ of vision. We are, moreover, justified, from an 

 d, priori consideration, in supposing that in depths to which no 

 sunlight descends, and which are illuminated by phosphores- 

 cent light only, peculiar organs of vision would have been 



