33« A HISTORY OF FISHES 



them have reached the Middle Atlantic, and have grown to 

 fifty to fifty-five millimetres. They finally arrive off the coasts 

 of Europe v^hen fully grown, about three inches in length, 

 and a little over two years old. They are now ready to undergo 

 the metamorphosis, and this takes place in the autumn. The 

 larvae cease to feed, the needle-like teeth are lost, and a pro- 

 gressive shrinking takes place both in length and depth, until 

 they assume a cylindrical, although still perfectly transparent 

 form, about two and a half inches long. These Elvers or Glass 

 Eels at once acquire a fresh set of teeth, small and conical, and 

 quite unlike those of the larvae, and are ready to commence 

 the ascent of the rivers. Considering this account of the larval 

 history with that of the breeding of the adults given on page 290, 

 it will be seen that the remarkable life-story of the Eel may 

 be divided into four chapters. These are: (i) a pelagic larval 

 stage — a period of active growth and passive migration; (2) the 

 metamorphosis into the Elver; (3) the growth of the ordinary 

 Yellow Eel; and (4) the change into the breeding Silver Eel, and 

 the migration to the spawning-ground, which ends in death. 



The true Flat-fishes {Heterosomata) are distinguished from all 

 other fishes by having both their eyes on the same side of the 

 head : the upper or eyed side only is coloured, the lower or blind 

 side being white. The larval history and metamorphosis of 

 these fishes is very remarkable, and throws considerable light 

 on the evolutionary history of the group. The eggs are pelagic 

 and are hatched in a very few days. The newly hatched larvae 

 are quite symmetrical, with an eye on each side of the head 

 as in any other fish, and they swim at or near the surface of the 

 sea. After a time, when the larvae have grown to half an inch 

 or more in length, one eye moves round to the upper edge of the 

 head and finally round to the opposite side, where it comes to 

 lie close to its fellow; at the same time, the dorsal fin is prolonged 

 forward, and as soon as the eye has moved round the fin grows 

 along the edge of the head above it. In some species the 

 migration of the eye is delayed until the dorsal fin has grown 

 forward on to the head, and the eye is then obliged to push its 

 way through between the base of the fin and the edge of the 

 head. While these important changes are taking place the little 

 fish sinks to the bottom of the sea, and thereafter lies or swims 

 at or near the bottom with the eyed side uppermost. The 

 twisting of both eyes to the one side of the head leads to radical 

 changes in the symmetry of the skull, and it is of great interest 

 to note that, if the migration of the eye is the effect of habit, as 



