10 ZOOLOGY 



such as sugar or salt, with which they are contrasted. 

 The terms refer to states or conditions rather than to 

 substances, but a number of important facts are con- 

 nected with the colloidal nature of the living material. 

 Guyer defines protoplasm as an aggregate of colloids 

 holding water for the most part, and in this water crys- 

 talloids are held in solution. Now such colloids may 

 be more or less solid or liquid, and when they reach the 

 more solid condition they are spoken of as gels. Pro- 

 toplasm, in the living state, undergoes such changes, 

 and these are reversible, so that the two states may 

 alternate indefinitely. It is supposed that many of 

 the visible phenomena of the cell are due to conditions 

 of gelation. Heat or poisons may act so severely on 

 the protoplasm that an irreversible gelation results, 

 when death at once ensues. We are reminded of elastic 

 substances which lose their elasticity on being subjected 

 to too severe a strain. 



irritability 9. One of the most striking properties of protoplasm 

 and stimuli j g j ts irritability; that is, its power of responding by 

 movement of some kind to a stimulus, which may 

 arise externally or internally. This disturbance or 

 stimulus may be physical or chemical, but the essential 

 point is that the living material does not merely trans- 

 mit the wave of energy, as an iron bar may transmit 

 heat, but displays characteristic movements of its own. 

 Irritability, in a biological sense, includes all such re- 

 sponses. Thus, if you meet a friend, who smiles in 

 response to your greetings, he is exhibiting irritability 

 in the sense now employed. In the lower forms of life 

 this irritability is more or less general, and the paths 

 of disturbance are indefinite; but in higher animals 

 there is a definite nervous system, and through it 

 messages are very rapidly transmitted to and from the 



