350 GEOLOGY. 



by Beecher (Fig. 158), made by combining parts of different indi- 

 viduals of Triarthus beckii, shows that this species possessed a row 

 of legs extending along each side of the body and that these were fur- 

 nished with fringed natatory branches. From these appendages, it 

 is inferred that the Triarthus both ran and swam and was a swift active 

 animal. It possessed antennae, which probably signifies a well-developed 

 tactile sense. The swimming filaments have not been found in some 

 genera where the legs have been observed. In these cases it is noted 



Fig. 158. — Triarthus beckii Green, showing the locomotive and other appendages. 

 Dorsal and ventral aspects. (After Beecher.) 



that the legs are relatively weak, and that these are forms which had 

 the habit of rolling up, while the Triarthus was stiffly articulated. 

 The suggestion has hence been made that the trilobites followed two 

 lines of development. The one class, being weak in power of loco- 

 motion and sluggish in habit, developed a jointed articulation and 

 found safety when attacked in rolling up armadillo fashion, while the 

 other acquired effective running and swimming organs and found 

 safety in flight; but present data are insufficient to demonstrate this 

 distinction, for the appendages of most trilobites are still unknown. 



