166 THE OCEAN 



arctic waters. Some authors regard these 

 two species as identical. 



The gigantic coral reefs and islands of the 

 ocean, as well as the dead shells making up 

 the globigerina and pteropod oozes of the 

 ocean-floor, present excellent evidence of the 

 metabolic activity of animals and plants in 

 the warmest waters of the globe. The living 

 mantle of organic matter over the surface of 

 the coral reefs may not at any moment be 

 very large in amount, but the presence of the 

 reefs, and the rate at which the calcium 

 carbonate skeletons are laid down, clearly 

 indicate a much greater metabolic activity 

 than is to be found anywhere in the Arctic 

 or Antarctic seas. In the cold water of the 

 deep sea there are among benthonic organ- 

 isms, on the other hand, no large molluscan or 

 other calcareous shells. The largest mollusc 

 shell dredged by the "Challenger" in deep 

 water was about six and a half inches in 

 length and was thin and transparent. 



Viscosity. — Still another condition depend- 

 ing on temperature is the viscosity of the 

 sea- water (see Chapter V.), which plays an 

 important role in the development of suspen- 

 sion-organs in plankton animals as well as 

 plants. In the surface waters of the tropics 

 a temperature of 80 °F. is found, and the vis- 

 cosity is, as we have seen, only one-half of 



