THE ORIGIN OF MAN 31 



dermata that arose early in the Ordovician time. 

 They are similar to crinids in their general con- 

 struction but they differ in that blastids have no 

 arms, only delicate armlets called brachioles. 

 These are situated at the sides of the five large 

 and conspicuous ambulacral areas on the sides of 

 the calyx and which are never seen in crinids. 



During the early Silurian the invertebrates, 

 which were the corals, crinids, bryozoans, brachio- 

 pods and trilobites, still dominated the seas for as 

 yet the fishes were not at all prominent. The 

 Middle Silurian was very similar to the Lower. 

 In the Upper, fishes first made an appearance 

 but as they are very much like those of the De- 

 vonian, we will discuss them later. The scorpions 

 of the Silurian are the oldest known air-breathing 

 animals and probably had their origin in the 

 eurypterids of the sea. But were these scorpions 

 wholly adapted to dry land, or did they live along 

 the seashore between the tides? Their remains 

 are rarely found in marine deposits, but usually 



