84 THE VITAL FUNCTIONS 



through the contractile vacuole. It will be noted 

 that these substances are not nearly so fully oxidised 

 as CO 2 , i.e. oxygen forms a much smaller proportion of 

 the molecule, and the animal derives a negligible 

 quantity of its energy from the breaking down of 

 proteins. Plants have no comparable nitrogenous kata- 

 bolism. If katabolites are formed from the breaking 

 down of proteins they are not systematically excreted, 

 but are probably often used again for the formation of 

 proteins. Plants are, in fact, much more economical of 

 their nitrogen than are animals. Various substances, 

 some of them containing nitrogen, may be cast off 

 by plants, as in the bark of trees, while carbon dioxide 

 regularly diffuses out of the tissues that are not green, 

 and from all tissues in darkness, but there is in plants 

 no specialised excretory system. 



(8) Movement. This is the most obvious of all the 

 expressions of life. We can again distinguish between 

 the movement of the protoplasm of living cells such as 

 we see in the streaming of the protoplasm in amoeba 

 and the movement of parts (for instance the limbs, 

 involving whole organs and systems of tissues in the 

 higher animals), or the movement of the whole organism 

 from place to place (locomotion). In amoeba we can 

 observe under the microscope how the first leads directly 

 to the second (formation of pseudopodia), and this 

 again to the third. In a higher animal the same con- 

 nexion occurs, but the chain of causation is much longer. 

 The movements of parts, for instance the limbs, depend 

 on the contraction of the living substance of highly 

 specialised tissues, the muscles ; through the definite 

 relationship of these with the bones the movements 

 of several muscles, co-ordinated through the agency of 

 the central nervous system, leads to the movement of 



