DIATOMACE^E 197 



stipitate forms occur attached in slimy masses to 

 Algae, &c., and the free forms as a fine yellowish- 

 brown scum on muds, or floating free in the surface 

 and subsurface waters of the ocean, both coastal 

 waters and those of the open sea. Just as the 

 Rhabdospheres are tropical, and the Coccospheres 

 and Peridiniece characteristic of temperate tracts of 

 the ocean, the Diatoms are found in greatest abun- 

 dance in the colder waters of the northern and 

 southern oceans, though they occur in all seas. 

 They greatly outweigh all other pelagic plants in the 

 colder waters. Sir James Ross's Antarctic Expedition 

 discovered a great tract of ocean bottom in the 

 southern ocean composed of diatomaceous ooze, i.e. 

 the siliceous valves of dead Diatoms. Here, as else- 

 where, the Challenger Expedition discovered an 

 enormous abundance of living Diatoms in the 

 surface tow-nets. South of latitude 50 the tow-nets 

 were sometimes so filled with Diatom scum " that 

 large quantities could be dried by heating over a 

 stove, when a whitish felt-like mass was obtained." 

 As this surface life perishes, the dead siliceous walls 

 are rained down on the bottom to form the diatom- 

 aceous ooze. 



They are found abundantly in shell-fish and 

 Crustacea, which use them as food, and also in the 

 digestive tracts of fishes, and they undoubtedly are 

 the most important ingredient in the vegetable 

 pastures of the sea which support the whole of the 

 animal life in it. They are very abundant in guano, 

 into which they have entered from the intestinal 

 canals of birds living on marine animals. 



