THE ORIGIN OF MAN 47 



and could not be folded back over the abdomen 

 as in modern insects. The thorax had three seg- 

 ments, followed by a long and slender abdomen 

 in which the segments were alike, ending in two 

 long appendages connected with the breathing 

 organs, as in the living may flies. Their eyes 

 were compound and their antennae simple. 

 These primal, which are thought to have arisen 

 from the trilobites, gave rise to several transition- 

 al stocks, which in turn changed into the modern 

 insects, such as the dragon flies, cockroaches and 

 grasshoppers. 



The scorpions and many forms of rather stout 

 spider-like animals, having a distinct cephalo- 

 thorax (head-trunk) and usually a large ab- 

 domen are found in this period. Thousand-legs 

 or Myriapoda are plentiful in this period but 

 there is no evidence of land snails until the Mid- 

 dle Pennsylvanian. 



The Amphibia are most common in the Penn- 

 sylvanian and their origin goes back to at least 



