THE CHAIN OF LIFE TRACED BACKWARD 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 Palaeozoic, 

 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 leading 

 groups of these persistent invertebrates under a few 

 general names, we may use the following, begin- 

 ning with those highest in rank : — 



(i) Insects and Crustaceans (ArthrOPODA). 



(2) CuttleSy univalve and bivalve Shell-fishes 

 (MOLLUSCA). 



(3) Worms (Annelida). 



(4) Sea - urchins and Sea - stats (ECHINODER- 

 mata). 



(5) Coral Animals, Sea- anemones, and Sea- 

 jellies (Ccelenterata). 



