CHAPTER IV 



FROM THE SIMPLE CELL TO THE COMPLEX ANIMAL 



45. 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 appar- 

 ently 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 remain- 

 ing together. Owing both to internal and external forces the 

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

 ferences among- themselves which 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 complex 

 condition in thfe adult animals is not a sudden one, but takes 

 place very gradually and the work which was at first 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 specializing of certain 

 cells to do each part of it, and the differentiation of structures 

 makes it possible to do each separate task better than before. 

 Thus differentiation of parts and division of labor go hand in 

 hand as we pass from the simple to the complex animals. 



29 



