SURVEY OF UNICELLULAR ANIMALS 



49 



gether so that the shell becomes essentially an internal struc- 

 ture. The almost incalculable number of Foraminifera constitutes 

 an important source of marine food for small animals which, 

 in turn, are the food of economically important fishes. The more 

 resistant Foraminifera shells sinking to the sea-bottom cover vast 

 areas with the so-called Globigerina ooze, accumulation of which 

 in the geologic past is evidenced to-day by the chalk cliffs of Eng- 

 land. The Pyramids and the Sphinx are built of Foraminiferous 

 rock. (Figs. 20, 244.) 



The final two orders of the Sarcodina, known as the Heliozoa 

 and Radiolaria, are characterized by unbranched, radiating 

 pseudopodia, each supported by a core of more dense protoplasm. 

 Most species are floating, spherical forms and the pseudopodia, 



< / 



LI '/ // 





■■- ■ 



m 





y 



Sv 



Fig. 21. — A, Heliozoon, Actinophrys sol; B, Radiolarian, Thalassicolla 



nucleata. (From Kudo.) 



protruding from the entire surface of the cell, give the appearance 

 of the conventional figure of the Sun. Accordingly the fresh-water 

 forms are commonly called Sun Animalcules, or Heliozoa. The 

 Radiolaria are all marine and are more complex than the Heliozoa, 

 the protoplasm being differentiated into several layers, enclosing 

 a dense, perforated central capsule, and usually supported by 

 an elaborate skeleton of silica. The Radiolaria vie in numbers and 

 importance with the Foraminifera in the economy of the sea; the 

 deposits of their skeletons forming the Radiolarian ooze. Cer- 

 tain rock strata hundreds of feet thick are contributions to the 

 Earth's surface made by the Radiolaria during bygone ages. 

 (Fig. 21.) 



