394 



FISHES 



CHAP. 



cylinders, each terminating in a strongly convex cornea (Fig. 

 227). 1 The eyes are directed either upwards or forwards, 

 and, as their long axes are parallel in either position, it 

 is probable that these Fishes are capable of binocular vision. 

 In the young of certain Teleosts occurring in the Antarctic 

 and Indian Oceans the large eyes are situated at the ex- 

 tremities of extraordinary long stalks extending from the sides 

 of the head. 



In the quasi -parasitic Cyclostome, Myxine, and in many 

 Teleosts belonging to widely different families, which live at 



great depths in the sea or 



scl cp.lh.aX inhabit subterranean waters, 



the eyes suffer from disuse 

 and degenerate in structure. 

 The influence of a deep-sea 

 habitat on the eyes of Fishes 

 is somewhat varied. The 

 eyes are often small. A few 

 abyssal Fishes are totally 

 blind, and no external trace 

 of eyes can be seen (Fig. 

 430). In such Fishes com- 

 pensation is often afforded 



FIG. 226. Vertical section of the eye of Salmo . , . 



fario (semi-diagrammatic), arg, Argentea ; OJ an extraordinary develop- 



ch, clioroid ; ch.gld, choroid gland ; en, ment o f tactile organs in 

 cornea ; cp.hal, campanula Halleri ; ir, iris ; 111 



I, lens ; opt.nv, optic nerve ; pg, pigmentary the form 01 long barbels, Or 



layer ; ^rtf, processus falciformis ; r<, retina; o f trailing filaments derived 

 set, sclerotic. (From Parker and Haswell.) 



from the median or the paired 



fins (Fig. 371, B). Many deep-sea forms possess eyes of the 

 normal siz3, or even exceptionally large eyes, probably because 

 either they occasionally migrate towards the surface, or else they 

 possess phosphorescent organs and are able to see by the aid of 

 the light they themselves emit. A blind Siluroid (Amiurus 

 nigrilabris] frequents the cave streams of Pennsylvania, and 

 many members of the same family which live in muddy waters 

 have very small or even minute eyes. One of the Gobies (Typhlo- 

 gobius}? which buries itself in the sand, or is found under stones in 

 the holes of a burrowing Crab on the coast of California, is also 



1 Chun, Aus den Tiefen des Weltmeeres, Jena, 1900, p. 534. 

 - Ritter, Bull. Mus. Comp. Zool. xxiv. 1 893, p. 51. 



