THE SAND-EEL FAMILY. 



811 



every stage of its eventful career. Of what nature is the strong 

 instinct which causes the post-larval plaice to migrate down- 

 wards and the post-larval sand-eel to work its way upwards we 

 cannot say ; we do not even know the immediate factor through 

 which the instinct works out its ends. A change in the specific 

 gravity of the fish, a migration of the fish-food upwards or 

 downwards, a sensitiveness to light, all suggest themselves, as 

 possibilities, but nothing more. At the length of six inches 

 the lesser sand-eel becomes sexually mature ; and the life- 

 cycle is thus completed. 



The early changes in habitat of the spring-larvae are indi- 

 cated graphically in the accompanying diagram. 



Length of 

 Fish. mm. 



Jam. Feb. march Apr. May June July 

 B = bottom. M = mid- water. 



Aug, Sept. Oct. Nov. 

 S = surface-water. 



Note. In explanation of the above diagram, the area between 1 on left and 

 2 on right hand indicates the second year ; while from 2 on left to 3 on right i3 



