THE SAND-EEL FAMILY. 



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. 

 no 



165 

 160 

 155 

 150 

 145 

 I4O 

 135 

 130 

 125 

 120 



115 



Zx 

 110 



105 

 100 

 85 



8SZ 



80 



7B 



7020 



66 



60 



55 



>* 



7o 



50 

 45' 



35 

 30 

 25 

 20 



IS 



10 



5 



ja 



a 



<a S 





g 



xi 



JAN. FEB. MARCH APR. MAY JUNE JULY AUG. SEPT. OCT. Nov. DEC. 



03 



a 3 



02 



01 



B = bottom. M = mid-water. 



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 is 

 the area of the third year. 



