CELLS AND TISSUES 241 



interior of the animal. Respiration takes place all over 

 its surface and there is a constant circulation of the living 

 substances. 



As we pass up the scale to an animal such as the fresh- 

 water Hydra we find digestion carried on in the interior 

 cavity of the body; there are definite organs, the tentacles, 

 for the capture of food; there are muscle fibers which by 

 their contraction change the form of the body; and there are 

 special nettling cells set apart for the purpose of protection. 

 Nevertheless respiration is carried on, not by special or- 

 gans, but over the entire surface of the body; all parts of 

 the body apparently eliminate waste products; there is no 

 blood, nor are there organs of circulation. Special or- 

 gans have been set apart for some kinds of work, while 

 other functions are discharged by the body in general 

 much as in the Amoeba. 



When we come to higher animals there are organs es- 

 pecially fitted for respiration, such as the gills of fishes 

 and the lungs of mammals; special organs are adapted 

 for excretion, such as the kidneys; and other organs are 

 exclusively concerned with the circulation of the blood. 

 Passing up the scale of life more and more organs are added ; 

 each becomes especially fitted for its work and at the same 

 time less able to do the work of other parts. 



The organs of a higher animal are formed of different 

 kinds of materials called tissues, and most tissues are in 

 turn composed mainly of small bodies called cells which 

 bear somewhat the same relation to the organism as a 

 whole as bricks bear to a brick house. Cells are masses of 

 protoplasm, commonly, though not always, surrounded 

 by a membrane or cell wall, and containing a small vesicle 

 known as the nucleus. The latter structure is an essential 

 element in the life of the cell. Cells multiply by a process 

 of fission, the nucleus dividing along with the protoplasm 



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