28 ANIMAL BIOLOGY 



flow around them, and when they have been digested within the 

 cell, the protoplasm moves onward leaving the waste material 

 behind. In many other unicellular animals, such as Paramecium, 

 an active, internal circulation, or cyclosis, of the protoplasm is 

 visible, and the cells of certain multicellular plants, such as Nitella 

 and Tradescantia, afford remarkable exhibitions of rotation and 

 circulation of protoplasm. 



Moreover, locomotion in Paramecium and related Infusoria is 

 effected by myriads of short, vibratile, thread-like prolongations 

 of the cytoplasm, termed cilia; while other unicellular forms, such 

 as Euglena, move by long whip-like processes, or flagella. Fi- 

 nally, ciliated cells form membranes, or ciliated epithelia, that 

 serve various purposes in the bodies of higher animals. Thus the 

 food and respiratory currents of Molluscs, such as the Clam, are 

 induced by ciliary action, and certain internal passages of the 

 human body are provided with cilia for the transport of materials. 

 (Figs. 6, 7, 11, 22, 27.) 



These and all other reactions of living matter are results of its 

 irritability and involve not only response to a stimulus but also 

 conduction so that the cell, or group of cells, as a whole is directly 

 or indirectly influenced — a condition which attains its fullest ex- 

 pression in the higher animals with a nervous system. Every 

 organism responds as a coordinated unit — an individual. It adapts 

 itself structurally and functionally to the necessities of its existence. 

 This power of adaptation, as exhibited in active adjustment be- 

 tween internal and external relations, overshadows every mani- 

 festation of life and contributes, more than any other factor, to 

 the enormous gap that separates even the lowest forms of life from 

 the inorganic world. 



The characteristics which we have described — specific organiza- 

 tion, chemical composition, metabolism involving the power of 

 maintenance, growth, and reproduction, and irritability resulting 

 in the power of adaptation — individually and collectively are 

 characteristic of living matter. Any formal objections that may 

 be raised to the diagnostic character of one or another only serve 

 to emphasize the unique conditions which obtain in life. 



In our discussion thus far, we have endeavored to describe the 

 characteristics of life in terms of its organizational units — cells, 

 and of its physical basis — protoplasm. But we have not attempted 



