PART II 



STRUCTURE AND CLASSIFICATION OF 



ANIMALS 



CHAPTER XI 



THE ELEMENTS OF STRUCTURE 



1. The resemblances and contrasts between plants and animals 2. 

 The relation of the simplest animals to those which are more 

 complex 3. Structural analysis of the animal body : organs, 

 tissues, cells, and protoplasm 4. Organs and organelles 5. 

 Classification of organs 6. Correlation of organs 7. Homology, 

 analogy, and con . ergence 8. Change of function 9. Substitu- 

 tion of organs 10. Vestigial organs 11. Adaptation in Organs 



STRUCTURE and function, or form and habit, are com- 

 plementary facts of life, in reality inseparable. For 

 scientific purposes they may be studied apart, and the 

 study of form and structure (the static relations of the 

 organism) is called Morphology, while the study of habit 

 and function (the dynamic relations of the organism) 

 is called Physiology. Before passing to think of animals 

 as regards their structure, it may be useful to compare 

 them with plants. One reason for doing this is to make 

 clear that the two great kinds of organisms on the 

 whole so very different- -have a deep structural resem- 

 blance, for the bodies of both are built up of cells and 

 transformations of cells. 



1. The Resemblances and Contrasts between Plants 

 and Animals. Every one could point out some differ- 

 ences between a tree and a horse, but many might be 

 puzzled to distinguish clearly between a sponge and a 

 mushroom, while all have to confess their inability to 

 draw a firm line between the simplest plants and the 



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