142 THE OCEAN 



of coral reefs, other species encrusting rocks 

 and protecting them from erosion. They 

 date back at least to the Jurassic period, and 

 in Tertiary times they built up a large part 

 of certain geological formations. 



Floating Algce. — Turning now to the pelagic 

 algae making up the phytoplankton, we find 

 that these, unlike the attached algae along 

 the coasts, are all small, the majority being 

 of microscopic dimensions. They are found 

 floating everywhere within the photic zone in 

 countless myriads, being, however, most 

 abundant in the subsurface layers of this zone, 

 and are of the greatest importance in the 

 economy of the sea, for, with the exception of 

 the fixed algae and matters carried down from 

 the land by rivers, they build up all the organic 

 substances upon which marine animals depend 

 for food. The same rule holds in the sea as 

 on the land, that all animal life depends 

 directly or indirectly on vegetable life. 



The marine phytoplankton includes green, 

 blue-green, and brown algse. The vast 

 majority belong to the brown algae, the blue- 

 green algae being represented only by the 

 Oscillatoriaceae, and the green algae only by 

 Halosphcera, 



Brown Algce (Phaeophyceae) include diatoms, 

 peridineans, coccolithophoridae, and xan- 

 thellse. 



