ANIMAL LIFE 161 



become less numerous farther out to sea. 

 In tropical and sub-tropical regions there 

 are many overgrown larval forms (Plagusia, 

 Phyllosoma, Alima, Erichthus, etc.), which 

 are supposed to have been carried by currents 

 from the shallow waters near shore into the 

 open ocean, where they could not find 

 congenial conditions for their development 

 sexually, and hence have grown to a great size. 

 It is still maintained by some authors that 

 the Leptocephalus of the common eel is a 

 larva of this kind. If it be quite normal, it 

 has the peculiarity of being reduced in size 

 on passing from the leptocephalid to the elver 

 staofe. The salmon runs into rivers and lakes 

 to spawn, but the fresh-water eel puts on a 

 silvery coat and descends to the ocean to 

 spawn, though as yet its eggs have not been 

 found. The youngest larval stages known 

 were taken in the Sargasso Sea by the 

 " Michael Sars " in 1910. 



The assemblage of pelagic or planktonic 

 organisms within the photic zone of the 

 ocean is self-sufficient and self-supporting. 

 The minute plants, like peridineans and 

 diatoms, build up organic substances by 

 the aid of chlorophyll and sunlight, and 

 furnish food for such creatures as cope- 

 pods and amphipods, which in turn are 

 food for fishes, birds and whales. Here we 



