i88 



A HISTORY OF FISHES 



and as they lie parallel to one another, it is possible that these 

 fishes are capable of binocular vision. In the rare and curious 

 oceanic fish {Opislfioproctus) the telescopic eyes are directed 

 upwards, and cannot be turned in any other direction. In the 

 young of other oceanic forms {Stylophthalmus) the eyes are 

 placed at the end of very long stalks growing out from the 

 sides of the head (Fig. 74c). The Hammer-headed Sharks 

 [Sphyrna), with the eyes placed at the extremities of lobe-like 

 lateral outgrowths (Fig. 74a), have been already described 

 in an earlier chapter. 



In a large number of Bony Fishes there is a distinct connection 

 between the habitual mode of life and the degree of perfection 

 of the organs of vision. In the Cat-fishes (Siluroidea) , for example, 



^outA 



^.// 



OJoe^i^o 



Fig. 74E. 

 Opisthoproctus soleatus, X ^. 



and in other forms living in more or less turbid water the 

 eyes are much reduced in size and efficiency, and in others 

 (Cyprinids, Cat-fishes, Cyprinodonts, etc.), which have taken 

 to a life in caves, wells, or subterranean streams, these organs 

 have disappeared altogether, although the young may be born 

 with well-developed and perhaps functional eyes (cf. p. 232). 

 In the Hag-fish (Myxine), which is in the habit of burrowing 

 into the body of a living fish and devouring its flesh, the eyes 

 are quite vestigial. Among oceanic fishes the eyes vary greatly 

 in size, and biologists have found considerable difficulty in 

 attempting to explain the connection between the size and 

 efficiency of the eyes and the intensity of the light at diflferent 

 depths. A trawl coming up from the abyssal depths will 

 contain fishes with large eyes, side by side with others totally 

 blind or with very small eyes. Starting at the top, fishes living 



