CHAPTER XXXIII. 



ZOOLOGY GENERAL AND STRUCTURAL. 



THE science of zoology treats of everything relating to ani- 

 mals. It considers their external form, anatomical structure and 

 physiological processes, their geographic distribution and eco- 

 nomic value. It attempts to trace out the life-history of animals 

 from the lowest to the highest forms; it considers also their re- 

 lations to one another, to the members of the vegetable kingdom, 

 and to other forms of environment. 



The animal body is made up of about the same elements found 

 in the plant, but the proportions are different; in animals the 

 albuminous and calcium compounds predominate. From these 

 compounds the different tissues of the body are formed, and the 

 tissues variously modified and combined constitute the organs of 

 the animal body, which may be divided into the organs of the 

 mechanical system, the organs of digestion, circulation, respira- 

 tion and excretion, the organs of reproduction and of the nerv- 

 ous system with those of the special senses. To trace the devel- 

 opment of these different groups of organs and to follow out the 

 processes carried on by them is an important part of the work 

 of zoology. 



The living portion of the animal, as of the plant that which 

 moves, assimilates food and grows is protoplasm, and an indi- 

 vidual portion of protoplasm, called a cell, is the unit of all ani- 

 mal structures. Thecell consists of a small portion of protoplasm 

 which is frequently surrounded by a structureless membrane. It 

 usually contains a nucleus, which may contain a nucleolus. The 

 nucleus may take different forms, but it always contains the nu- 

 clear fluid and the nuclear substance, each of which is of a proto- 

 plasmic nature. 



Free, isolated cells exist in the blood, chyle and lymph. Those 



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