CHAPTER XXII 



THE ANCESTORS AND INTERRELATIONS OF THE 



VERTEBRATES 



The purpose of this chapter is to point out the probable re- 

 lations between the vertebrates and invertebrates, to unify our 

 account of the vertebrates by discussing the interrelations of 

 the class, and to indicate the extent of our knowledge concerning 

 the ancestors of vertebrates secured by the* study of fossil forms. 



i. The Relations between Vertebrates and 

 Invertebrates 



A problem that has commanded the attention of many emi- 

 nent scientists has been to trace the ancestry of the vertebrates 

 to some invertebrate form. Investigations along this line have 

 resulted in a number of theories, each with many adherents 

 ready to argue in its favor. It is impossible in this place to give 

 an account of each of these theories, but that their differences 

 are considerable may be inferred from the fact that scientists 

 have derived the vertebrates from the annelids, nemerteans, 

 insects, arachnids, flatworms, and echinoderms. 



The origin of vertebrates from the echinoderms through the 

 Enteropneusta (p. 386, Fig. 332) and Amphioxus (p. 394, Fig. 

 341) seems to have so many points in its favor that this theory 

 will be sketched briefly in the following paragraphs as an illustra- 

 tion of the method used in tracing vertebrate descent. 



We have seen that there are a number of subphyla in the 

 phylum Chordata that contain animals of a lower grade than 

 the vertebrates. These are: (1) the Enteropneusta (Figs. 

 332-336), which includes a few worm-like species; (2) the Tuni- 



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