CHAPTER XXVI 



INTRODUCTION TO THE VERTEBRATES 



Vocabulary 



Specialization, development of parts for special function. 



Survival, remaining alive. 



Ultimate, furthest. 



Vertebrates, animals having a back bone composed of vertebrae. 



While it is certain that all living things are more or less related 

 to each other, still they have developed along very different lines, 

 and to very different extents. 



Among animals, the protozoa seem to have carried the specializa- 

 tion of the single cell about to its limit, which, while assuring their 

 survival, could not possibly raise them very high in the scale of 

 development. 



The sponges have obtained the utmost possible advantage from 

 colonizing slightly specialized cells in unspecialized bodies; and 

 have attained a considerable advance over the protozoa. 



The hydra and its relations reached a much higher plane by 

 development of tissues for special purposes and among them first 

 appear the three-layered body wall from which the organs in higher 

 animals are derived. 



The worms mark a very diverse class but some of them have 

 well-developed systems of organs, digestive, circulatory, nervous, 

 etc., which had never appeared in previous forms. 



Diverging from the worm type it seems as if nature had tried 

 out several schemes of development, carrying each to a point where 

 it could no longer be much improved. 



The molluscs represent the ultimate advantage to be gained 

 from a protective shell and rather high internal development, 



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