ORGANISM AND ENVIRONMENT 3 



be borne in mind by the student in using the table of classification 

 which will be found as the Table of Contents at the beginning of 

 this book. 



In surveying the diverse organisms which constitute the Inverte- 

 brata, the student should bear in mind the following principles. 



The most fundamental of the characteristics of living organisms is 

 the way in which, in the face of an environment which presents as 

 many dangers as opportunities, they hold their own by making ad- 

 justments within themselves. This statement applies equally to the 

 struggle for existence of the individual and to the slow racial adjust- 

 ments which we know as evolution. 



The term environment^ is a collective name for all the external 

 things which affect any living being. Four principal factors constitute 

 the environment — the ground or "substratum" (if any) upon which 

 the organism stands, the "medium" (water or air) which bathes it, 

 the heat and light which it receives from the sun's rays or can lose to 

 its surroundings, and the other organisms in its neighbourhood. Of 

 these factors the substratum has in most cases relatively little im- 

 portance, and we may dismiss it now. 



The medium, on the other hand, is of enormous importance. Meet- 

 ing all parts of the surface of objects that it contains, it exerts every- 

 where a pressure upon them, supports them, may transport them, 

 affects the movements they execute, and controls all exchange, 

 whether of matter or of energy, between them and the world about 

 them ; and from it animals obtain their supplies of free oxygen, often 

 of water, and sometimes of food. If it be liquid, according as the 

 concentration of substances dissolved in it be greater or less than that 

 within the organism water and solutes will tend to pass to or from the 

 body of any animal which is not covered by an impermeable cuticle. 

 This exchange is of the utmost importance, both as a danger by up- 

 setting equilibria within the body and as an advantage by facilitating 

 the excretion of substances which are harmful in the organism. It is 

 controlled by the surface layer of protoplasm, which either is, or 

 owing to surface tension behaves as, a delicate membrane that has the 

 power of actively regulating, to some extent, the passage of substances 

 through itself, and by the activity of the organs of excretion. If the 

 medium be gaseous, according to the amount of water vapour it 

 contains water will tend to evaporate to it from the surface of the body. 

 This is important owing to the necessity for the intake of water by the 



^ The student may occasionally be puzzled by the phrase "internal en- 

 vironment". This bizarre contradiction in terms is sometimes applied to what 

 we shall presently call the "internal medium" (p. 132). 



