124 



THE ORIGIN AND EVOLUTION OF LIFE 



itive shrimp-like form, Burgessia hella (Fig. 22), a true crusta- 

 cean, which may be compared with Apus lucasanus, a mem- 

 ber of the most nearly allied recent group. We observe a 

 close correspondence in the shape of the chitinous shield (car- 

 apace), in the arrangement of the leaf-like locomotor appen- 

 dages at the base of the tail, and in the clear internal impres- 



MEROSTOMATA 



RUSTACEA 



Fig. 22. Horseshoe Crab and Shrimp, Cambrian and Recent. 



Molaria spinifcra, a mid-Cambrian merostome (after Walcott), compared witli the 



recent "horseshoe crab," Limiilus polyplicmus. 

 Btirg<:ss:a bclla, a shrimp-like crustacean of the Middle Cambrian (after Walcott), 



compared with the very similar Apus lucasanus of recent times. 



sions in Burgessia of the so-called "kidneys," with their 

 branched tubules. The position of these organs in Apus is 

 indicated by the two light areas on the carapace. Other 

 specimens of Burgessia found by Walcott show that the taper- 

 ing abdominal region and tail are jointed as in Apus. 



The age of the armored merostome arthropods is also 

 thrust back to mid-Cambrian times by the discovery of several 

 genera of Aglaspidas, the t}qDical species of which, Molaria 

 spinijera Walcott, may be compared with that "living fossil," 



