CHAPTER THIRTY-SIX 



PHYLUM ARTHROPODA 



Class Crustacea 

 Subclass Trilobita 



THE trilobites, now entirely extinct, were formerly Thetrfio- 

 very abundant in the sea. They were highly developed ^^ t n 



as early as Cambrian time, group 

 The body was segmented, 

 and bore very numerous 

 jointed appendages. It is 

 possible that these primi- 

 tive Crustacea gave rise to 

 some centipedelike type 

 which took to the land, 

 developing tracheae or air 

 tubes for breathing. It 

 has been suggested that in 

 this manner the trilobites 

 may have been remote an- 

 cestors of the insects ; but 

 Dr. G. C. Crampton, of the 

 Massachusetts Agricultural 

 College, has lately given 

 good reasons for excluding 

 them from the direct line 

 of ancestry. At the time 

 when the insects were becoming dominant the trilobites 

 were disappearing. 



Subclass Eucrustacea (or Crustacea proper) 



The appendages are modified in various ways, for Appendages 

 locomotion, for feeding, and as organs of sense. Conse- 



257 



FIG. 70. A trilobite, Dalmanites, show- 

 ing the dorsal surface. 



