THE HISTORY OF ANIMAL LIFE 401 



Protozoa, Porifera, Coelenterata, Annelida, Molluscoidea, Mollusca, 

 Echinodermata, and Arthropoda of the early eras having left re- 

 mains of types that with rare exceptions differ in important respects 

 from modern types. Later records show successive appearance of 

 more complex representatives of these phyla and the rise of new 

 types, consummated by the appearance of the Vertebrata, and, in 

 still more recent times, the class Mammalia. While the order of 

 origin of many invertebrate phyla is lost in the far-distant past, it is 

 inferred from the order of origin of the more complex animals 

 in these phyla that the simplest animals were first to appear. 



Fossil remains of the shell-forming Protozoa, the Foramenifera 

 and Radiolaria, abound in certain limestone formations. In fact, 

 some limestones are composed of countless billions of these tiny 

 shells. All Foramenifera are marine animals; the occurrence of 

 their fossils in any region indicates that that region was at some 

 time a part of the ocean bed. Fossil Porifera are also common in 

 some rock formations; having spicules of lime or of silica, they are 

 preserved in numbers. The Coelenterata are represented by fossil 

 corals; soft-bodied coelenterates have left only imprints in the rocks. 

 Worms such as Annelida have left their tubular burrows. Of the 

 Echinodermata the most interesting fossils are those of the class 

 Crinoidea, once the most numerous of the phylum but now repre- 

 sented by comparatively few types. The Molluscoidea are chiefly 

 represented by the shells of the members of the Brachiopoda. The 

 most common type of fossils ordinarily met with are those of 

 Mollusca. Many common limestones are made up almost exclusively 

 of the calcareous shells of these animals. One has only to examine 

 samples of crushed limestone to find numerous varieties more or less 

 perfectly preserved. Fossil remains of Arthropoda are rather com- 

 mon. Imprints of insects have been found in which the wing spread 

 was more than two feet. It rather staggers one's imagination to at- 

 tempt to interpret what life must have been like when forms so 

 outside our daily experience existed. 



