THE NATURAL HISTORY SURVEY. 



175 



Besides the antennae, the cephalon bears four pairs of \hese 

 pediform, biramous appendages, the protopodites of which are 

 developed into large gnathobases which function as manducatory 

 organs. In the higher crustaceans these cephalic appendages 

 have been greatly differentiated as special organs for serving 

 the mouth, but in the trilobites they still function in part as 

 organs of locomotion, and except in the strong development 

 of the gnathobases they do not differ materially from the thor- 



FIGURE a^Triarthus becki, Green. A, dorsal, and B, ventral aspect 

 (x2). After Beecher. 



acic appendages and those of the pygidium, which are wholly 

 locomotive in function. The endopodites are jointed, ambula- 

 tory or crawling legs; posteriorly, especially upon the pygidium, 

 the joints become flattened and more or less leaf-like, carrying 

 tufts of setae, and are more adapted for swimming. The pro- 

 fusely fringed exo^odites also function as swimming organs, 

 although they serve in common with the entire membranous 

 ventral surface, as organs of respiration. 



