RESPONSIVE LIFE OF ORGANISMS 441 



escape our observation; but the constant differences of 

 behavior of a single organism when hungry and when well 

 fed must be due to stimuli arising out of its bodily states. 

 When well fed, it is less active and always less responsive to 

 external stimuli of every sort. A far more striking example 

 is offered by the cyclic conjugating reaction of Paramcecium 

 (see page in), which occurs at long intervals. Two com- 

 plemental individuals come together for exchange of 

 nuclear substances. These cycles suggest the breeding 

 periods of the higher animals, during which the maturing of 

 the sex cells occasions internal stimuli which call forth a 

 whole train of activities that are collectively known as 

 "breeding habits." 



So the protozoa, although their acts are few in kind and 

 simple in performance, exhibit a set of elemental reactions 

 of the most fundamental, wide spread, and comprehensive 

 sort. 



2. Some general features of the sensory mechanism of 

 the Metazoa. 



Protozoan and metazoan are alike organic wholes. Each 

 has developed about itself a protecting outer wall, sequester- 

 ing itself from the outer world. Each has created an inter- 

 nal world, within which all its vital processes are 

 carried on through the concordant action of part upon part. 



Whatever organs a protozoan may develop, it is limited 

 in this ; that they must all be developed out of the parts of a 

 single cell. In the metazoans, on the contrary, there are 

 many more or less independent, structural units, that can be 

 differentiated for separate functions. One set of cells may 

 be made to serve as receptors for stimuli (end organs); 

 another may be specialized for doing the contracting (muscle 

 fibers), and another (nerve cells and their processes or 

 fibers), may be fitted for intercommunicating between the 



