XXII. THE VERTEBRATE ANIMALS 



Increasing Complexity of Structure and of Habits in Plants and 

 Animals. — In our study of biology so far we have attempted to 

 get some notion of the various factors which act upon and interact 

 with hving things. We have learned something about the various 

 physiological processes of plants and animals, and have found them 

 to be in many respects identical. We have examined a number 

 of forms of plants and have found all grades of complexity, from 

 the one-celled plant, bacterium or pleurococcus, to the complicated 

 flowering plants of considerable size and with many organs. So 

 in animal life the forms we may have studied, from the Protozoa 

 upward, there is constant change, and the change is toward greater 

 complexity of structiu-e and functions. A worm is simpler in 

 structure than an insect, and in many ways, especially by its actions, 

 shows that it is not so high in the scale of life as its more lively 

 neighbor. 



We are already awake to the fact that we, as living creatures, 

 are better equipped in the battle for life than our more lowly neigh- 

 bors, for we are thinking 

 creatures, and can change 

 our surroundings at will, 

 while the lower forms of 

 animals are largely con- 

 trolled by stimuli which 

 come from without; tem- 

 perature, moisture, light, 

 the presence or absence of 

 food, — all these result in 

 movement and other re- 

 actions. 



In structure we also differ. Particularly is this difference seen 

 in the skeleton. We call ourselves vertebrates, because we have a 



274 



Cross section through (I) an invertebrate ani- 

 mal and (V) Si vertebrate animal: a, food tube; 

 h, heart; c, vertebrate column; n, central nerv- 

 ous system. 



