ECOLOGICAL 



8i 



many miles from the sea. But the flounder must return to its native 

 salt water to spawn, and the sea is also the scene of the larval life 

 and of the metamorphosis. Fully formed young flounders ascend 

 the rivers. 



The researches of Schmidt show that the chief breeding area of 

 the European Eel (Anguilla vulgaris) is in deepish water in the 

 south-western part of the North Atlantic, somewhat to the north 

 of the West Indies. Thence the transparent larvae (Leptocephali) 

 gradually make their way, helped by currents, towards the European 

 coasts, including those of the Mediterranean and the Baltic. As 

 they approach these coasts, being over two years and a half old, 

 and having traversed, it may be, two thousand miles, they undergo 

 a remarkable metamorphosis, from a knife-blade-like to a slender 



Fig. 1 8. 



Life-history of Salmon. From specimens, i, The egg; 2, the developing 

 embryo within the egg; 3, the newly hatched alevin, with yolk-sac 

 protruding; 4, 5, subsequent alevin stages; 6, the young fry, having 

 absorbed the yolk-sac. All approximately natural size. 



cylindrical shape, and are known as elvers. These ascend the rivers 

 and often pass to lakes, feeding voraciously and growing quickly, 

 continuing their nutritive life for five to eight years, the females 

 taking longer to assume the characters of maturity. The physio- 

 logical restlessness that then sets in prompts a migration down the 

 rivers and out to sea, the long journey taking several months. The 

 contrast to the avian scheme of migration is noteworthy, since the 

 adult eels seem to die after spawning, so that there is no return 

 journey to the fresh waters save for the next generation. Similarly 

 the large marine lamprey (Petromyzon marinus), ascends the rivers 

 to spawn and succumbs soon afterwards. The larvae, known popu- 

 larly as "niners", technically as "Ammocoetes", remain larvae for 

 three years or so, and then, attaining to adult characters, they go 

 down to the sea to put on flesh. The basal fact in the interpretation 

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