GEOGEAPHICAL DISTRIBUTION OF PLANTS AND ANIMALS. 



57 



Oceanic species include a large part of Diatoms or silica-secreting microscopic plants. 

 They live near the surface, requiring light like other plants, and are forced to keep near 

 the surface by the bubble of oxygen 



they give out in assimilation. They ^'^• 



abound especially in the southern At- 

 • lantic, and great areas over the bottom 

 are covered with a Diatom ooze or soft 

 mud. 



The Foraminifers, or calcareous 

 Ehizopods, are solely salt-water spe- 

 cies. They in part live near the sur- 

 face, if not altogether. These abound 

 in many seas, excepting the more frigid, 

 and make by their accumulation at the 

 bottom the Globigerina ooze, even to 

 depths of 174,000 feet. Maury, al- 

 luding to the dropping to the ocean's 

 bottom of the Foraminifers, says : 

 ' ' The sea, like the snow-cloud with 

 its flakes, in a calm is always letting 

 fall on its bed, showers of microscopic 

 shells, and all pelagic life adds to the 

 showers." 



Radiolarians, or siliceous Ehizo- 

 pods, occur only in salt water. They 

 are abundant in some localities in the 

 central Pacific, at a depth of 15,000 

 feet and less. They make a Radio- 

 larian ooze. A Radiolarian deposit on 

 the Barbadoes is supposed to indicate 

 an elevation of the sea-bottom of 1000 

 to 2000 feet. 



Siliceous Sponges occur in the 

 ocean at various depths to 15,000 feet, 

 and from warmer temperatures to 

 40° F. The Hexactinellids are most 

 abundant at depths of 80 to 100 fath- 

 oms, at which depth the Euplectella 

 (Fig. 29) was obtained near the Philip- 

 pines, in waters at a temperature of 

 59° F., and near Cebu, of 69° F. For 

 figures of a number of Sponge Spicules 

 see page 432. The Choristid Sponges 

 occur down to 16,200 feet, and the 

 Lithistids to 900 feet. The Sponges 

 with calcareous spicules or skeleton 

 are also widely distributed. Both the 

 calcareous and siliceous also occur in 

 shallow waters, fresh and salt. 



The ordinary or the Actinozoan 

 Corals are all marine. Solitary kinds 

 extend to great depths, and one species, Bathyactis symmetrica, has a vertical range 

 from 180 to 17,400 feet (Moseley). They are, therefore, species of all temperatures and 



Euplectella speeiosa, or Glass Sponge. 



