22 THE LESSON OF EVOLUTION 



on the mud or sand on which they have been thrown 

 by the waves, and when the umbrella is turned 

 upside down, the gastric cavities get filled with the 

 mud or sand and leave star-like marks, which are 

 easily recognised. The Actinozoa are represented 

 only by the curious Archceocyathince, which appear 

 to be related to the perforate corals. 



The hingeless Brachiopods were the first of their 

 class to appear. According to Professor C. E. 

 Beecher, Paterina of the Lower Cambrian 

 approaches nearest to the primitive stock, for it 

 closely resembles the embryonic shell of later forms ; 

 and during the Cambrian period it gave rise to at 

 least five different types of Inarticulata , one of which 

 represented probably by Kulorgina led up to the 

 Articulata, the first known of which, Orthis, had 

 become well established in the Upper Cambrian. 



Trilobites form by far the most important part of 

 the fossil fauna, and can generally be distinguished 

 by the shortness of the pygidium. Some were as 

 much as eighteen inches in length. The minute 

 Protocaris of the Lower Cambrian is a Phyllopod 

 with a subquadrate carapace. Marine Arachnida 

 are represented by Aglaspis. 



The Cambrian mollusca are remarkable for the 

 proportionately large number of elongated shells 

 formerly classed as Pteropoda. Lately, however, 

 many biologists have been led to the conclusion that 

 the Pteropoda have had a comparatively late origin. 

 It is now generally allowed that the Hyolithidce, 

 which includes all the pre-Cambrian and Cambrian 

 forms, do not belong to the Pteropoda. They must, 



