CHAPTER IV. 

 FROM THE SIMPLE CELL TO THE COMPLEX ANIMAL. 



44. The Individual as a Cell-composite. In the simplest 

 animals, as the Protozoa, the individual consists of a single 

 cell, and the life history of the individual animal is such as 

 has already been seen to belong to the cell (Chapter III). In 

 such an individual one cannot speak of organs in the ordinary 

 sense, for organs as we shall see are made up of cells bound 

 together in the doing of certain work. Yet it is important to 

 remember that there are none of the necessary duties of life, 

 such as getting food, digesting it, breathing, moving, repro- 

 ducing, and the like, 'which are not well done by these simple 

 one-celled animals. The many-celled animals agree with the 

 simpler ones in that they too start life as single cells ap- 

 parently quite as simple as the one-celled animals themselves. 

 When the cells divide, however, the daughter cells do not 

 separate as in the Protozoa, but form a mass of cells by cling- 

 ing together. Owing both to internal and external forces 

 the cells in the mass do not long remain alike, but soon show 

 such differences among themselves as serve as the basis for 

 the great variety of structures found in the bodies of the 

 higher animals. The change from the simple cell to the com- 

 plex condition in the adult animals is not a sudden one, 

 but takes place very gradually and the work which was 

 formerly done by the single cell is divided up among the 

 groups of different cells composing the body. The division 

 of the work to be done makes possible and necessary the spe- 

 cializing of certain cells to do each part of it, and the differen- 

 tiation of structures makes it possible to do each separate task 

 better than before. Thus division of labor and differentiation 

 of parts go hand in hand as we pass from the simple to the 



complex animals. 



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