148 THE OCEAN 



like shields, sometimes with a central spine, 

 and are generally limited to the warmer areas 

 of the ocean. These calcareous shields are 

 called coccoliths and rhabdoliths, and were 

 known from deep-sea deposits long before 

 the living organisms, coccospheres and rhab- 

 dospheres, were discovered by the " Chal- 

 lenger " in the surface waters. They have 

 been met with in geological deposits as ancient 

 as the Cambrian Period, showing that they 

 have retained their shape practically unaltered 

 through long ages. These rhabdospheres and 

 coccospheres may pass through the finest tow- 

 nets, and for a long time some naturalists 

 did not believe in their existence. They were 

 first detected during the " Challenger " Expe- 

 dition, entangled in the protoplasmic threads 

 of pelagic foraminifera and radiolaria and 

 in the stomachs of salpae and pteropods, and 

 are now collected plentifully by means of the 

 centrifuge from water-samples obtained at 

 different depths by the water-bottle. 



In the Arctic and Antarctic seas the cocco- 

 spheres are replaced by species without cal- 

 careous shields, such as Tetraspora {Phceocys- 

 lis) poucheti which occurs in enormous floating 

 brnks. 



(4) Xanthellce. — Besides the coccolitho- 

 phoridae there are other minute brown algae 

 in the surface waters of the tropical regions. 



