CHAPTER XXVIII 



THE PAST HISTORY OF INVERTEBRATES AND THE 

 BEGINNINGS OF THE VERTEBRATES 



I wrote the past in characters 



Of rock and fire the scroll, 



The building in the coral sea, 



The planting of the coal. 



Emerson, Song of Nature 



Vertebrates and Invertebrates. However various in form 

 and structure the members of the phyla thus far discussed 

 are, they have in common at least this negative character, 

 that in none of them has a backbone been developed. For 

 this reason they are collectively termed Invertebrates (Lat. in, 

 : 'not" ; vertebratus, "vertebrate") ; the animals which have 

 a backbone are called Vertebrates. It will be worth our while, 

 before beginning the study of the vertebrates, to consider 

 some general questions of interest in connection with the 

 evolution of the invertebrate phyla, and then to describe 

 briefly some peculiar forms which appear to stand between 

 the invertebrates and the vertebrates. 



The Invertebrates of Past Ages 



Sources of Information. There are three sources of in- 

 formation which the zoologist may draw upon in his en- 

 deavor to discover the relationship which the animals of the 

 past and the present bear to each other throughout the 

 long series from the lowest to the highest. These sources of 

 information are the geological record of species, compara- 

 tive anatomy or morphology, and the stages in the develop- 

 ment of the individual. We shall consider first the record of 



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