The Organs of Sense 119 



pear on the same side. This side is turned uppermost as the 

 fish swims in the water or when it hes on the bottom. This 

 distortion is a matter of development. The very young flounder 

 swims with its broad axis vertical in the water, and it has one 

 eye on either side. As soon as it rests on the bottom it begins 

 to lean to one side. The lower eye changes its axis and by de- 

 grees travels across the face of the fish, part of the bony inter- 

 orbital moving with it across to the other side. In some soles it is 

 said to pass through the substance of the head, reappearing on 

 the other side. In all species which the writer has examined 

 the cranium is twisted, the eye moving with the bones ; and the 

 frontal bone is divided, a new orbit being formed by this division. 

 In most northern flounders the eyes are on the right side in the 

 adult, in tropical forms more frequently on the left, these 

 distinctions corresponding with others in the structure of the 

 fish. 



In the lowest of the fish-like forms, the lancelet, the eye is 

 simply a minute pigment-spot situated in the anterior wall of 

 the ventricle at the anterior end of the central nervous system. 

 In the hagfishes, which stand next highest in the series, the eye, 

 still incomplete, is very small and hidden by the skin and mus- 

 cles. This condition is very different from that of the blind 

 fishes of the higher groups, in which the eye is lost through 

 atrophy, because in life in caves or under rocks the function of 

 seeing is no longer necessary. 



The Organs of Hearing. — The ear of the typical fish consists 

 of the labyrinth only, including the vestibule and usually three 

 semicircular canals, these dilating into sacs which contain one 

 or more large, loose bones, the ear-stones or otoliths. In the 

 lampreys there are two semicircular canals, in the hagfish but 

 one. There is no external ear, no tympanum, and no Eustachian 

 tube. The ear-sac on each side is lodged in the skull or at the 

 base of the cranial cavity. It is externally surrounded by bone 

 or cartilage, but sometimes it lies near a fontanelle or opening in 

 the skuU above. In some fishes it is lirought into very close 

 connection with the anterior end of the air-bladder. The latter 

 organ it is thought may form part of the apparatus for hearing. 

 The arrangement for this purpose is especially elaborate in the 

 carp and the catfish families. In these fishes and their relatives 



