24 THE COMMERCIAL SEA FISHES OP 



tactile organs ; in such instances, appendages for feeling, 

 as barbels, are more developed than eyes. Others, again, 

 are restricted to living in waters in dark caves, where rays 

 of light hardly enter. Irrespective of the foregoing in- 

 stances where atrophy occurs, there are many forms re* 

 siding in the dismal abysses of the ocean where solar light 

 is unable to penetrate. Another modifying influence is, 

 whether the habits of the fish are diurnal or nocturnal, the 

 latter, unless residing in dark places, requiring the largest 

 organs of vision. 



The eyes of fishes are mostly situated in orbital cavities, 

 where they rest upon a cushion of adipose or gelatinous 

 substance. Their direction is subject to considerable varia- 

 tion, for, although usually placed laterally, as in the perch, 

 they are not invariably so. The angler (Lophius) has them 

 on almost the upper surface of the head ; the hammer- 

 headed shark (Zygcena) has the sides of the head laterally 

 prolonged, at the extreme edges of which the eyes are 

 placed. The blennies appear to observe objects as well 

 with their heads out of the water as when submerged in 

 the sea, and they move their eyes independently one of the 

 other. In the sun-fishes (Orthagoriscus) there exists a 

 circular palpebral fold provided with a sphincter ; while 

 some sharks have a nictitating membrane. Among the flat 

 fishes are remarkable modifications, which I have alluded 

 to when treating of the species. 



I do not propose giving a description of the globe of the 

 eye and its various membranes, but simply to note a few 

 deviations from what commonly occurs. The sclerotic is 

 even found bony in some well-ossified forms ; the cornea is 

 nearly flat. The iris is a thin contractile curtain situated 

 behind the transparent cornea and suspended in the 

 aquecus humour ; its dilating and contractile powers are 



