PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 71 



evolved organs. Consciousness, for example, an attribute of the brain 

 and central nervous system in general, is not seen as such in the 

 protozoa, but its prototype irritability, with the coordinated responses 

 to stimuli, is common to every protozoon, and such stimuli sometimes 

 lead to reactions on the part of the protozoon which are often appar- 

 ently directed toward a given end until we are tempted to interpret 

 them as conscious acts. While most of the actions of protozoa are 

 reactions to external stimuli, many are combinations of reactions that 

 do not lend themselves to analysis. Such, for example, is the apparent 

 choice of food or of building material for shells and tests, or the com- 

 plex reactions that are frequently involved in the avoidance of some 

 obstruction. Not infrequently such reactions have been interpreted 

 as evidence that the protozoon acts wilfully, or with a certain amount 

 of intelligence of the end to be accomplished, and they are frequently 

 cited as examples of conscious activity on the part of these primitive 

 forms. Many of these so-called conscious acts can be explained by 

 the ordinary physical laws of fluids, and while one cannot deny that 

 the protozoon's actions may be conscious, it seems much more prob- 

 able that these activities are the fundamental, often physical or chemi- 

 cal, reactions which serve in evolution as the starting point for the 

 infinitely more complex activities which we call our consciousness. 



In all animals there is a certain amount of work clone in the daily 

 life, and the energy put into such work comes from the oxidation, or 

 physiological burning, of the body protoplasm. There is, therefore, 

 a constant waste of protoplasmic material which goes off as work 

 done, as heat, or as residual waste matters comparable with the smoke 

 and ashes of physical combustion. Such waste is made good by the 

 addition of new raw materials in the form of food, which is made over 

 into new protoplasm. The phenomena of waste and renewal are 

 usually spoken of together under the name of metabolism waste as 

 destructive, repair as constructive, metabolism. Food getting, there- 

 fore, becomes the first necessity of the living thing, and the chief end 

 toward which the fundamental structures of the body are directed, and 

 this, whether in the highest mammal or the lowest protozoon, becomes 

 the chief economic problem to be solved (Fig. 21). 



The methods employed by different kinds of living things are widely 

 varied, and the great problem is apparently well solved in many dif- 

 ferent ways. Green plants are the starting point for all living things, 

 for they manufacture not only their own food, but indirectly the food 

 for all other living things. This they are able to do because of the 

 chlorophyl or green colored matter which they possess and which has 

 the power to utilize the energy of sunlight in reducing CO 2 and manu- 

 facturing starch out of water and carbon. The further changes of the 

 starch into more complex substances, and these into protoplasm of 

 the plant, are buried in the obscurity of unknown chemical processes 



