THK CHAIN OF LIFE TRACED iUCKWAIU) II 



reptiles, reach somewhat farther, and the fishes 

 and the air-breathing arthropods farther still. On 

 the other hand, our six great groups of marine 

 invertebrates run back for a vast length of time, 

 without any companions, to the lowest Paheozoic, 

 and this applies to their higher types, the cuttles 

 and their allies, and the crustaceans, as well as to 

 the lower tribes. Turning now again to our table, 

 we find that these creatures extend in unbroken 

 lines back to the Lower Cambrian, the oldest beds 

 in which we find any considerable number of or- 

 ganic remains, and leave all the other members of 

 the Animal Kingdom far behind. 



If now we endeavour to arrange the leadin*^ 

 groups of these persistent invertebrates under a few 

 general names, we may use the following, begin- 

 ning with those highest in rank : — 



(1) Insects and Crustaceans (Arthropoda). 



(2) Cnttles, univalve and bivalve Shcll-lishcs 

 (MOLLUSCA). 



(3) Worms (Annelida). 



(4) Sea ' urchins and Sea - stats (Eciiinoder- 

 mata). 



(5) Coral Animals, Sea- anemones, and 6"^- 



jellics (CCELEN TERATA). 



