igo A HISTORY OF FISHES 



When it is considered that a fish such as the Trout {Salmo 

 trutta) will show a preference for a particular kind of fly, whether 

 natural or artificial, and will seize this with great rapidity 

 when it is placed within his range of vision, it seems certain 

 that, in some fishes at least, the sense of sight is a keen one. 

 A Trout has been known to refuse a certain type of fly again 

 and again, but when another was substituted of similar size 

 and shape, but of diflferent colour, this was promptly accepted. 

 This would suggest that the Trout is sensitive to colour, but 

 whether this is the general rule among fishes is not yet clear. 

 Experiments have shown that the sense of sight probably plays 

 the most important part in the search for food, but, at the same 

 time, this is much more hmited than that of a land vertebrate; 

 and, owing to the general haziness of the water, due to the 

 presence of organisms and other matter suspended therein, 

 objects must appear of somewhat uncertain outline. The 

 extreme convexity of the lens of the eye points to the fact that 

 a fish is near-sighted, and even in the clearest water it is 

 doubtful whether the range of vision exceeds about twelve yards, 

 if as far as this. It is not unhkely that the fish really notices 

 movements or changes in outline rather than actual objects. 



In describing the brain, mention was made of the pineal 

 gland or epiphysis arising from the roof of the primary fore- 

 brain. This is particularly well developed in the Sharks, but 

 in these and the Bony Fishes it is little more than a nervous 

 enlargement. In the Lampreys and Hag-fishes, however, this 

 structure bears a strong resemblance to an eye, and the external 

 skin covering this region is partially transparent in the adult. 

 In the fossil remains of ancestral Cyclostomes there are indica- 

 tions of the presence of one or two such median sense organs 

 on the upper surface of the skull, and it is possible that these 

 early fishes had one or two functional eyes on the upper surface 

 of the head in addition to those at the sides. 



The auditory organ or inner ear of a fish, the next of the sense 

 organs to be considered, consists of a membranous sac or 

 vestibule enclosed in a chamber or capsule on either side of the 

 hinder part of the skull. Its purpose is twofold, for not only is 

 it the seat of the sense of hearing, but it is also concerned 

 with the maintenance of equihbrium: indeed, it is possible 

 that the latter function is the more iniportant of the two. 

 Comparing the fish's ear with that of a man or other higher 

 vertebrate several important diflferences are at once apparent. 

 The human ear consists of three parts : the external, middle, 



