Amphipoda of the Caspian Sea. . 87 



crassus, G. Gregrokowii, G. portentosits, G. coronifer, G. 

 thaumops. Pandora ca?ca, Iplrigeneia abgssorum, Gammaracan- 

 thus caspius, and Amathilinella cristata), all of which are 

 furnished with eyes, but in very different degrees of develop- 

 ment : thus Gammar acanthus caspius has very large round 

 eyes, Gammarus coronifer and Amathilinella cristata long 

 but narrow eyes, Gammarus thaumops triangular unpigmented 

 eyes, and Pandora cceca small unpigmented eyes, which can 

 hardly be endowed with the faculty of sight. A still better 

 example is furnished by the following new Amphipoda dis- 

 covered by me in the Caspian Sea : — 



fathoms. 

 Onesimus caspius from the depth of 75-250 



pomposus „ ,, ISO 



platyuros „ „ 40-48 



Pantoporeia microphthalma „ „ 80-90 



Niphargus casjrius „ „ 35-90 



of which the last two species, together with Onesimus caspius } 

 were also taken in one cast, and, indeed, at a depth of 80- 

 90 fathoms, at 0° 36' E. long, and 41° 6' N. lat. Pantoporeia 

 microphthalma and Niphargus casp>ius possess pigmented but 

 small eyes ; of the species of Onesimus some possess red, others 

 (O. caspius) perfectly unpigmented eyes, which, in the last- 

 mentioned species at least, are deprived of the faculty of sight ; 

 and with these more or less blind species there live Mysidse, 

 the large, convex, and black eyes of which certainly absorb a 

 sufficiency of light even in the darkness of the depths. 



These examples may suffice to show that deep-sea existence 

 alone does not of necessity cause the retrogression of the 

 visual organ. Now, however, we will show from our Cas- 

 pian Amphipoda, how the animals stand related to the sea- 

 depths, how deep-sea existence acts upon their organization, 

 by what essentially the disappearance of the eyes is brought 

 about, and by what the latter are replaced in the event of 

 their retrogression. 



We may accept it as proved that with the increase of the 

 depth of the sea the quantity of rays of light diminishes, so 

 that at a certain distance from the surface the strength of the 

 light is very small, although it never falls to zero. But, 

 however weak the light may be, the possibility of vision is 

 not excluded, and the eyes of animals living in the abysses 

 need only to be adapted to the comparative darkness *. Such 



* I think it doubtful that absolute darkness commences at a depth of 

 100 metres, as found by Forel in the lake of Geneva; for I cannot at all 

 conceive of absolute darkness. I readily admit that at this or the other 

 depth the daylight no longer reacts upon certain chemicals ; but this does 

 not exclude the possibility of seeing. 



7* 



