LIFE AT AND NEAR SURFACE 123 



containing the creatures that live upon the 

 bottom of the sea, there are similar zones in 

 the water itself and each is inhabited by 

 certain forms of life that wander but little 

 above or below the depth to which nature has 

 adapted them. At the very surface of the sea 

 there live and thrive vast numbers of animals 

 which never descend to the bottom, even in 

 shallow water. Many of the creatures which 

 are found at some certain depth live in other 

 stages of their existence in entirely different 

 parts of the sea. The young of many shells, 

 crustaceans and other marine animals swim 

 freely about at the surface and only descend 

 to the bottom when they attain a definite 

 stage in their growth, while others live, dur- 

 ing their youth, upon the bottom and in after- 

 life rise to the surface of the sea and never 

 again descend to their birthplace. 



For this reason scientists are often greatly 

 puzzled and many years have been devoted to 

 studying what was considered a new form of 

 marine life, only to discover eventually that 

 it was but one stage of some well-known crea- 

 ture whose life-history had been hitherto un- 



