EVIDENCES— GEOLOGY 



135 



The outer branch of the appendage, or cxopodite, has a long 

 fringe which apparently converted it into a swinniiing and 

 respiratory organ. In all of these characters and in others, 

 the trilobite corresponds closely with fundaniontal crustacean 



Fig. 74. — A trilobite, Triarthrus hecki, restored. 1, dorsal aspect; 2, ventral 

 aspect. X 2. (From Lull, after Beecher.) 



characters which may be expected in ancestors of the latter 

 organisms. 



Aquatic Arthropoda and Their Descendants. On this basis the 

 Crustacea are judged to have arisen from the trilobites very 

 early in the Paleozoic. Some species long extinct show unmis- 

 takable similarity to the trilo])ites. One closely related group of 

 primitive Arthropoda, the Palaeostraca, is still represented bj^ a 

 single species, the horseshoe cral). This group included forms, 

 now extinct, from which developed the primitive Arachnida in 

 the Silurian. The class Arachnida persists in the nimierous 

 modern spiders, mites, ticks, scorpions, etc., all terrestrial 

 species. 



