GENERAL ZOOLOGY 



Summary 



In this survey of the invertebrate groups we have considered the vital func- 

 tions common to all, as well as the structural features Avhich the functions 

 involve and upon which they depend. The facts presented justify the general 

 conclusion that whereas all organisms have been confronted with a relatively 

 limited number of common problems of survival, in terms of vital func- 

 tions, there is an almost unlimited number of solutions to the problems in 

 terms of structural adaptations. In many groups of animals the structures 

 developed for certain functions are homologous; in other groups structures of 

 diverse embryonic origins have become adapted to serve common functions 

 and are thus analogous but not homologous. It is also apparent that there 

 are no discontinuities between vertebrates and invertebrates in functional 

 considerations. There are, however, so many more types of invertebrates 

 than of vertebrates that the range of structural adaptations possible among 

 invertebrates is infinitely more varied than among vertebrates. Perhaps we 

 should again emphasize the fact that the distinction between vertebrates 

 and invertebrates is chiefly one of convenience and of itself implies no 

 formal taxonomic differences. 



With this chapter we conclude our discussions of major invertebrate phyla 

 and turn to the phylum Chordata, which includes all the vertebrates as well 

 as a few minor groups of invertebrates. One additional phylum, the Hemi- 

 chordata, consists only of invertebrates, and this group is to be considered 

 along with the chordates, to which it is obviously closely related. 



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