CHAPTER XXII 



THE EVOLUTION OF INVERTEBRATES AND THE ANCESTRY 

 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 ; vertebratm, vertebrate) ; the animals which have a 

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

 before beginning the study of the latter, 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 inverte- 

 brates and the vertebrates. These intermediate forms, or their 

 ancestors, may be the immediate ancestors of the vertebrates. 



THE EVOLUTION OF THE INVERTEBRATES 



Sources of Information. There are, as we have seen in 

 the chapter on insects, three sources of information which the 

 zoologist may draw upon, in his endeavor to discover the rela- 

 tionship which the animals of the past and the present evi- 

 dently bear to each other, throughout the long series from 

 the lowest to the highest. These sources of information are 

 the geological record of species, comparative anatomy or 



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