134 THE STORY OF FISH LIFE. 



That is to say, they do not represent the back 

 and the belly, but the right and left sides. In 

 some fish it is the right side which is upper- 

 most, in some the left. When the fish swims it 

 does so by an undiilatory motion of the body ; 

 that is to say, progressing by means of wave-like 

 movements passing from head to tail. But it 

 does not swim vertically, but retains the position 

 which it holds when at rest — the dark side being 

 kept uppermost. Another point about the adult, 

 which we shall appreciate now, is the fact that 

 the eyes are not on opposite sides of the head, 

 but lie side by side on the upper surface. How 

 this comes to be, and how it is that the fish comes 

 to lie always on one side or the other, we may 

 discover from a study of the larval fish. This, 

 when it emerges from the egg, is perfectly sym- 

 metrical, and gives all promise of developing into 

 the typical fish-like form. Soon, however, a 

 change becomes obvious, for there is a marked 

 tendency to lie at rest on one side, right or left, 

 which becomes more and more pronounced daily. 

 Simultaneously v/ith this new position, the left 

 or right eye begins to migrate from what is now 

 fast becoming the under to the upper si<le, and 

 the attainment of this end is accomplished at the 

 expense of the symmetry of the skull, which 

 eventually, with the complete migration of the eye 

 to the upper surface, becomes quite asymmetrical. 

 The reason for the really wonderful transforma- 

 tion exhibited by the young flat-fish is one of 

 nature's mysteries which no one has yet succeeded 

 in solving. 



But eels and flat fish do not exhaust the list of 



