CHAPTEE III. 



THE STUDY OF ORGANS. 



The following is a short summary of the chief of the 

 different parts of a highly organized animal. Except 

 where the contrary is distinctly stated, the structure 

 described is that of a mammal, the highest form 

 of the vertebrates cr animals with backbones. It is 

 easier to bear in mind the parts of a more com- 

 plicated kind of animal, and then think of all the 

 less complicated kinds as leading up to this type, 

 than to learn the animal series from its simpler types, 

 and note the development of the different tissues and 

 organs as they appear one by one in the series. 



The lowest forms of animals, called the Protozoa, 

 are for the most part unicellular, or consist of com- 

 pound aggregates in which separate cells cannot be 

 distinguished. The rest of the animal kingdom, 

 which are possessed of multicellular bodies, are called 

 the Metazoa. Of these, the lower forms (Ccelenterata) 

 consist practically of two layers of cells, and are there- 

 fore spoken of as diploblastic animals ; while the 

 rest, possessing a more complicated structure, are 

 spoken of as triploblastic, or three-layered. (See 

 figs. 11, 12, p. 46.) The vertebrate is of course in- 

 cluded among the latter. 



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