DIATOMACE 197 
stipitate forms occur attached in slimy masses to 
Alge, &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 Peridiniee 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, 4.¢. 
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 00ze. 
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. 
