RUDIMENTARY EYES 



723 



It would seem, indeed, that all the inhabitants of this still, cold, dark world 

 should tend to lose their eyes ; possibly they would were it not for the 

 development of luminous organs, a common acquisition by the inhabitants 

 of the benthos. 1 



Thus among abyssal Molluscs {Chiton, etc.) the eyes tend to degenerate 

 even in Cephalopods wherein these organs are usually well marked ; the 

 only known blind C'ephalopod, however, is Cirroihaiima murrayi, an octopod 

 which inhabits the N. Atlantic at depths of approximately 3,000 metres 

 (Chun, 1911). Similarly among Crustaceans living at moderate depths, the 

 arrangement of the pigment surrounding the ommatidia of the compound 

 eyes remains permanently in the dark-adapted position, while in bathy- 



FiG. 866. — The Eye of a Blind Sel.\chian Fish, BEyrHOBATis. 



The ocular structures are of the most rudimentary t\-pe. BV, blood vessel 

 C, cornea ; Car, cartilage ; CC, connective tissue ; /, iris ; P. retinal pigment 

 R, retina (after Brauer). 



pelagic tj'pes various stages of degeneration appear wherein all pigment is 



absent {C ydodorijypef' or the ommatidia entirely disappear and the eye-stalks 



become fused with the carapace or are converted into tactile organs 



{Cymonomus, etc.) (Doflein, 1904). Paradoxically, side-by-side with species 



with degenerate eyes dwell other Crustaceans (shrimps, etc.) with fully 



developed and pigmented eyes, frequently, however, in creatures of a roving 



habit (Edwards and Bouvier, 1892). '^ In general among bathypelagic fishes, 



species which penetrate to lower and lower depths develop progressively 



better eyes, adopting all possible expedients to improve their vision in dim 



illumination — a telescopic shape, an immense lens, a huge pupil, a brilliant 



tapetum, and a multiplication of the rods — until these organs become 



relatively larger than in any other Vertebrate. But below 500 metres in 



many instances the struggle is given up and the eyes shrink so that among 



the deeply benthonic fishes they are often vestigial and functionless or 



1 p. 736. 2 p_ 166. 



3 Compare the " wondrous-eyed hopper " (Fig. 203), an inhabitant of the deep seas. 



