SECT. ii. DIATOMACESE. 205 



floating diatoms, that matter which diminishes their 

 specific gravity and makes the plant buoyant which 

 otherwise would be weighed down by its silicious coat. 

 At those periods in which the structural and reproduc- 

 tive phenomena proceed most vigorously, their position 

 in depth must be fluctuating ; hence they approach and 

 vanish from, the surface. Their growth is perfected by 

 the heat and light which penetrates the sea in calm 

 weather. 



Diatoms are social plants crowded together in vast 

 multitudes. Dr. Wallich met with an enormous as- 

 semblage of a filamental species of Khizoselenia, which is 

 from six to twenty times as long as it is broad, aggre- 

 gated in tufted yellow masses, which covered the sea 

 to the depth of some feet, and extended with little in- 

 terruption throughout six degrees of longitude in the 

 Indian Ocean. They were mixed with glistening yellow 

 cylindrical species of such comparatively gigantic size as 

 to be visible to the naked eye. 



Other genera constitute the only vegetation in the 

 high latitudes of the Antarctic Ocean. Dr. Hooker 

 observes, that without the universal diffusion of diatoms 

 in the South Polar Ocean, there would neither be food 

 for the aquatic animals, nor would the water be purified 

 from the carbonic acid which animal respiration, and the 

 decomposition of matter, produce. These small plants 

 afford an abundant supply of food to the herbivorous 

 mollusca and other inhabitants of the sea, for they 

 have been found in the stomachs of oysters, whelks, 

 crabs, lobsters, scallops, &c. Even the Noctiluci, those 

 luminous specks that make the wake of a boat shine 

 like silver in a warm summer night, live on the floating 

 pelagic diatoms, and countless myriads are devoured by 

 the enormous shoals of salpi and other social marine 

 animals. 



The silicious shells of the diatoms form extensive 



