u6 



The Organs of Sense 



be a nostril, a small depression at the front of the head, covered 

 by ciliated membrane. In the hagfish the single median nostril 

 pierces the roof of the mouth, and is strengthened by carti- 

 laginous rings, like those of the windpipe. In the lamprey the 

 single median nostril leads to a blind sac. In the Barramunda 

 (Neoceratodus} there are both external and internal nares, the 

 former being situated just within the upper lip. In all other 



fishes there is a nasal sac on either side of the head. This has 



I 



usually, but not always, two openings. 



There is little doubt that the sense of smell in fishes is rela- 

 tively acute, and that the odor of their prey attracts them to 



FIG. 83. Dismal Swamp Fish, Chologaster cornutus Agassiz. Supposed ancestor 

 of Typhlichthys. Virginia. 



FIG. 84. Blind Cavefish, Typhlichthys subterraneus Girard. Mammoth Cave, 



Kentucky. 



it. It is known that flesh, blood, or a decaying carcass will 

 attract sharks, and other predatory fish are drawn in a similar 

 manner. At the same time the strength of this function is yet 

 to be tested by experiments. 



The Organs of Sight. The eyes of fishes differ from those of 

 the higher vertebrates mainly in the spherical form of the crys- 

 talline lens. This extreme convexity is necessary because the 

 lens itself is not very much denser than the fluid in which the 

 fishes live. The eyes vary very much in size and somewhat in 

 form and position. They are larger in fishes living at a mod- 

 erate depth than in shore fishes or river fishes. At great depths, 



