50 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 



and much distorted toward the blind side, and the teeth are very 

 small or obsolete. 



The most remarkable of the structural changes which accompany 

 the sidewise modification of the body of the flat-fishes are the 

 changes which result in bringing the eye which normally belongs to 

 the side turned toward the bottom over onto the side that is exposed 

 to the light. Both eyes,- therefore, come to lie on the upper side of 

 the head; the fish thus loses its bilateral symmetry, and the head 

 acquires its peculiar twisted appearance. These changes are not 

 confined to the eye merely, but involve a considerable torsion of the 

 bones of the head, and an asymmetry of the nerves connected with 

 the displaced parts. These aspects of the asymmetry will be very 

 briefly referred to later, but for details, reference must be made to 

 special papers which deal with the osteology of the flat-fishes. 



The degree of asymmetry in the different species is an adaptation 

 correlated with their habits. Those fishes like the halibut, sand- 

 dab, and summer flounder, which are the more free-swimming, 

 and which feed to a considerable extent on other fishes, are the more 

 symmetrical; the displaced eye is nearer the line of the profile,. and 

 the mouth parts of the two sides are scarcely different. In the 

 forms like the winter flounder, which live more upon the bottom 

 and in the shallow water and which subsist largely on the bottom 

 invertebrates, and eat other fishes very little, the asymmetery 

 of the eyes has gone further, while the mouth has become 

 twisted toward the under side, and those portions of the jaws 

 which are on the upper side remain relatively undeveloped. It 

 is in the soles that the asymmetry of both eyes and mouth has 

 gone to its greatest extreme. These fishes live on the bottom almost 

 entirely, and probably obtain most of their food by grubbing in the 

 mud. The mouth in this species has become very much twisted. 

 From these facts it may be inferred that the asymmetry of the 

 mouth is independent and secondary to that of the eyes. The 

 asymmetry of the eyes is the result of the need of having both eyes 

 on the side toward the light, while the asymmetry of the mouth is a 



