THE LIFE OF THE SIMPLEST ANIMALS 19 
traced far back in the geologic history of the world. It is 
an ancient type of animal structure. 
The Radiolaria, too, which live abundantly in the pres- 
ent oceans, especially in the marine waters of the tropical 
and temperate zones, are found as fossils in the rocks from 
the time of the coal age on. The siliceous shells of the 
Fra. 11.—Heliosphera actinota (after HaecKEL); a radiolarian with symmetrical 
shell. 
Radiolaria sinking to the sea bottom and accumulating 
there in great masses form a radiolaria-ooze similar to the 
globigerinz-ooze ; and just as with the Globigerinz, the 
_ remains of the ancient Radiolaria formed thick layers on 
the floor of the ancient oceans, which have since been up- 
‘titted and now form certain rock strata. That kind of 
tock called Tripoli, found in Sicily, and the Barbados 
‘earth from the island of Barbados, both of which are used 
