CHAPTER V 



TISSUE-DIFFERENTIATION FOR SPECIFIC 

 FUNCTIONS 



I. DIFFERENTIATION IN ANIMALS 



IN the Protista cellular differentiation is identical 

 with differentiation of the organism. It would be 

 the same, so far as Paramecium is concerned, if each 

 cilium were a tiny vibratile cell instead of being, as 

 most biologists believe, a minute portion of the one 

 cell composing the body of the animal. We then 

 might speak of these locomotor organs as the 

 " ciliary system." In the many-celled animals and 

 plants the organism also acts as a unit, and the 

 differentiation of cells for the better performance of 

 specific functions such as we have traced in the 

 previous chapter is carried out apparently with ref- 

 erence only to the needs of the whole. The struc- 

 ture of the higher animals is so complex and the vari- 

 ous systems so specialized that, in studying them, 

 this point is often lost sight of. 



Alimentary System. The taking in of food is 

 the most necessary function of living things, and only 

 those forms, like the tapeworm, which have become 

 absolved from the responsibility of seeking food on 

 their own account are without a special apparatus 

 for seizing, storing, and reducing foods. In practi- 



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