THE ORIGIN OF MAN 21 



the waters swarmed with a vast variety of inver- 

 tebrate animals, chiefly of bryozoans, which were 

 exceedingly small animals resembling corals, 

 brachiopods, gastropods, cephalopods and trilo- 

 bites. It is thought that armored fishes came into 

 being during this period as an abundance of frag- 

 mentary vertebrate remains has been found which 

 are somewhat similar to the type seen often in 

 the late Silurian and Devonian times. The 

 faunas of the upper Ordovician were at first sim- 

 ilar to those of the Middle. But the Arctic in- 

 vasion introduced new types of animals. These 

 differences are seen more especially among the 

 bryozoans, brachiopods and corals. Before we 

 proceed with the Silurian period let us discuss the 

 animals with radial symmetry as among the lowly 

 organized invertebrates, which were the first 

 forms of life, there is a vast variety in which the 

 outer form and the arrangement of the organs are 

 not bilaterally planned as in other animals. The 

 study of these animals will therefore help us to 



