ORGAN OF SIGHT, 111 



voracious Serrasal monoids of the South American rivers. 

 There is no reason to doubt that the seat of that perception is 

 in the olfactory sac ; and it may be reasonably conjectured 

 that its strength depends mainly on the degree of develop- 

 ment indicated by the number and extent of the interior folds 

 of the pituitary membrane. 



Orgcm of Sight. — The position, direction, and dimensions 

 of the eyes of fishes vary greatly. Tn some they have an 

 upward aspect, and are often very close together; in others 

 they are lateral, and in a few they are even directed down- 

 wards. The Flat-fishes represent the extraordinary anomaly 

 that both eyes are on the same side of the head, and rarely 

 on the same level, one being generally placed more forward 

 than the other. In certain species of marine fishes the eyes 

 are of an extraordinary size, a peculiarity indicating that the 

 fish either lives at a great depth, to which only a small pro- 

 portion of the rays of light penetrate, or that it is of nocturnal 

 habits. In fishes which have descended to such great depths 

 that no rays whatever can reach them, or in fresh-water fishes 

 living in caves, or in species which grovel and live constantly 

 in mud, the eyes are more or less aborted, sometimes quite 

 rudimentary, and covered by the skin. In very few this 

 organ appears to be entirely absent. In some Gobioids and 

 Trachinoids (PcTiopMhalmus,Boleo2')ht]ialmus, Uronoscopus, etc.) 

 the eyes,wliich are on the upper side of the head, can be elevated 

 and depressed at the will of tHe fish. In the range of their 

 vision and acuteness of sight, Fishes are very inferior to the 

 higher classes of Vertebrates, yet at the same time it is 

 evident that they perceive their prey or approaching danger 

 from a considerable distance ; and it would appear that the 

 visual powers of a Feriophthalmus, when hunting insects on 

 mud-flats of the tropical coasts, are quite equal to that of a 

 frog. Again, the discrimination with which fishes sometimes 

 prefer one colour or kind of artificial fly to another affords 



