258 



THE LIFE OF CRUSTACEA 



begins. In spite of these disadvantages, however, 

 the study of fossil Crustacea does throw considerable 

 light on the evolution of the group, and it is likely 

 that interesting results in this direction await future 

 investigations. 



In the earliest fossiliferous rocks the most abundant 

 and important Arthropods are the Trilobites (Fig. 81), 



FIG. 81 RESTORATION OF A TRILOBITE (Triarthrus becki), SHOWING 

 THE APPENDAGES. UPPER SIDE ON RIGHT, UNDER-SIDE ON 

 LEFT. SLIGHTLY ENLARGED. (After Beecher.) 



an extinct group which appears to have been related 

 to the primitive Crustacea. The name Trilobite 

 refers to the three-lobed form of the body when seen 

 from the dorsal side, most species having a pair of 

 grooves running lengthwise which divide off a middle 

 lobe containing the principal organs of the body from 

 two lateral " pleural " expansions covering the limbs. 



