138 THE STORY OF FISH LIFE. 



like dilatations. The great length of these 

 fin-rays shows that these fish are hatched far 

 down in the depths of the sea, where absolute 

 stillness prevails, the currents such as prevail at 

 the surface would wreak ruin on such fragile 

 structures at once. 



A most remarkable trait in the life history of 

 larval fishes is that exhibited by the young of the 

 sand-smelts (Atkerina), which, for some time after 

 hatching, cling together in dense masses and in 

 enormous numbers. It is said, by the way, that 

 that peculiarly larval and archaic type, the 

 Amphioxus, occasionally forms a swimming chain 

 by uniting one with another by their mouths. 

 This is the only other instance I can recall com- 

 parable to the masses of young fry of the 

 sand-smelt. 



CHAPTER XL 



MIGRATION AND HYBERNATION 



THE migratory impulse seems to be as strongly 

 developed in the fishes as in the birds. In no 

 other vertebrates, indeed, save these two classes, 

 do we find these periodic movements so well 

 marked. This is probably due to the peculiar 

 facilities oifered either by air or water for ex- 

 tensive journeys under fairly uniform conditions. 

 Barriers such as confront non-flying terrestrial 

 animals being absent. 



Migration with both bird and fish is generally 

 associated with the provision for the next genera- 



