4 THE INVERTEBRATA 



mouth to compensate for it, and also because the latent heat of the 

 evaporation lowers the temperature of the body. Whether the medium 

 be liquid or gaseous, it offers, according to the free gases it contains, 

 varying possibilities of interchange of oxygen and carbon dioxide 

 with organisms. This has naturally extremely important effects upon 

 respiration. 



The loss or gain of heat tends, of course, to affect the temperature 

 within organisms, and with this the chemical processes of the latter 

 vary, being, as is usual in such processes, slowed as the protoplasm 

 becomes colder and quickened when it is warmed, and being brought 

 finally to a stop when its minute organization is destroyed either by 

 the coagulation of certain of its proteins by heat or by the freezing of 

 its water. Every organism is tuned to work within a range of tem- 

 perature peculiar to it. "Warm-blooded" animals keep their tem- 

 perature within proper limits by active chemical and physical means; 

 "cold-blooded" animals (to which all invertebrata belong) are in this 

 respect at the mercy of their surroundings except in so far as they 

 can circumvent them by their habits. Light, while it is essential for 

 photosynthetic organisms, has chemical effects of importance in many 

 others, and in all which possess sense organs capable of appreciating 

 it is a source of stimuli from the external world. 



Relations between an animal and other organisms in its surround- 

 ings are almost always based in the long run on nutrition. Either such 

 organisms serve the animal for food, or they attack it to make it their 

 food, or they are competitors for a common food-supply, or in rarer 

 cases they assist it, or obtain its assistance, in the quest for food or in 

 defence against enemies which would use it or them for food. Only 

 between members of opposite sexes of the same species are there 

 relations of another kind, namely those which are concerned with 

 reproduction. The coming of organisms into relation with one another 

 usually involves the receipt of stimuli and more or less complicated 

 behaviour, with the use of organs of locomotion and prehension. 



The action of the environment upon the organism will be seen to 

 be threefold: (i) it affects it mechanically, as by transporting it from 

 place to place, by the impact of adjacent objects, or by the attacks of 

 enemies ; (2) it affects the working of the living machine by the com- 

 pulsory introduction or abstraction of materials (water, salts, etc.) or 

 of energy ; (3) it directly stimulates it to activity, which may be an in- 

 evitable response, such as the movement of certain organisms towards 

 light (phototaxis) or be dependent upon conditions existing at the 

 moment in the organism ; or it may inhibit such activity. Besides such 

 action the environment may affect the organism negatively, by failure 

 in respect of food, oxygen, or some other necessity which the organism 

 is dependent upon obtaining from its surroundings. Where such 



