INTRODUCTION 15 



tractile power may be limited in other cases to parts of 

 cells, as in those forms which swim by the lashing of 

 slender projections known as flagella, or by the waving 

 of an animated nap or pile upon their surfaces, the cilia, 

 of which more will be said. In all cases of energetic 

 movement we feel justified in assuming that the source 

 of the power, is in destructive chemical reactions, and 

 that a draft is being made upon the fuel stores of the 

 cells. 



The Association of Cells. When many cells are massed, 

 as in the body of a worm, the situation of the single unit 

 differs significantly from that of the cell leading an inde- 

 pendent existence. First of all, its environment is made 

 for it to a great extent by other cells. A very small 

 minority are in direct contact with the outside world; 

 the great majority are submerged among their fellows. 

 The typical cell is, therefore, shut in from food supplies 

 of the casual sort on which the free-living cell depends. 

 It is remote from the oxygen of the surrounding air or 

 water. A cell so situated would perish were it not for one 

 of the most striking features of the larger organisms, a 

 moving liquid medium, which bathes the cells and acts 

 as a common carrier. This fluid supplies food and oxygen 

 and removes wastes. 



The cells composing the body of any animal are of a 

 common descent, but they have taken on widely different 

 characters and have become adapted to particular func- 

 tions. The cell which is in itself a complete living thing 

 must perform all the essential activities for itself the 

 preparation of crude food, locomotion, etc. In the 

 multicellular animal the individual cells have come to be 

 far more restricted in their powers. Many have become 

 passive structures serving only for mechanical support or 

 surface protection. Such cells may or may not be living. 

 Others, while clearly alive, have ceased to perform cer- 

 tain functions. With limited exceptions movement is 

 exhibited only by those systematically arranged cells 

 which form the contractile tissues. Almost all the cells 



