Phylum Arthropoda 509 



THE CLASS TRILOBITA 



During the Cambrian and Silurian periods of the Paleozoic era, 

 the trilobites (Fig. 169,A) were among the most abundant of the in- 



METATARSUS 



TRACHEAL 

 SPIHACLE 



Fig. 169. — A, Dorsal view of a trilobite; B, ventral view of female spider. 



