CAVE-FISH. 125 



One of the most curious of these fishes is the Anableps, or Four- Four- 

 eyed Fish, Anableps tetrophthalmus, 417, of the fresh waters of eyed Fish, 

 tropical America. This fish has not really four eyes, but its eyes, 

 which bulge considerably, are divided into an upper and a lower 

 portion in such a manner that the fish as it swims at the surface of 

 the water can see both in and out of that medium, the upper part 

 of the eye being adapted for use in air and the lower part for use 

 in water. There is nothing exceptional in the general appearance 

 of the large crystalline lens and the retina, but the iris is curiously 

 altered so as to present two pupils, one above the other. This is 

 effected by a pair of overlapping horizontal flaps of the iris passing 

 across the middle of the originally simple pupil. 



The Amblyopsidas are small fishes related to the Cyprino- Cave-fish. 

 dontidse. They are confined to North America; some live in 

 streams and ditches and have eyes, others occur in subterranean 

 waters of the great limestone caves. These last are blind, the eyes 

 being vestigial and hidden under the skin. The body is colourless 

 and transparent. One of the best known is Ambhjopsis spelaa (418) , 

 the Cave-fish or Blind-fish of the Mammoth Cave of Kentucky. 

 By way of compensation for the loss of vision the sense of hearing 

 is very acute, as also is the tactile sense, there being developed on 

 the head numerous transverse ridges with papillae, which judging 

 from their abundant nerve supply are delicate organs of touch. The 

 largest specimen known is five inches long. The fish is vivi- 

 parous, and the young are about a quarter of an inch long when 

 born. 



Physoclisti. 



In the grade Physoclisti the air-bladder, although developing Wall- 

 as a hollow outgrowth of the alimentary canal, becomes shut off case 10- • 

 from it by the disappearance of the neck or tube, the " ductus 

 pneumaticus." Some of the Berycoid fishes (Wall-case 12), 

 the most primitive of the Acanthopterygian fishes, are exceptional 

 in this respect. The great majority of the Teleostean fishes 

 belong to the grade Physoclisti ; the exhibited series extends 

 from the middle of Wall-case 10 along the North and East 

 Walls to the other end of the Gallery (Wall-case 20). 



