CHAPTER XIV 

 ANIMAL SENSE ORGANS 



Adjustments in Lower Animals. — In Chapter XII 

 the general fact of the sensitiveness or irritability of pro- 

 toplasm was stated, and the reaction of plants to stimula- 

 tion discussed. In that chapter the point was made that 

 in respect to fundamentals plant protoplasm and animal 

 protoplasm act similarly, the differences that exist de- 

 pending mainly on differences in the relation of the or- 

 ganisms to their food supply. Thus it comes about that 

 in plants the responses to stimulation take the form of 

 slow changes, due usually to growth, while in animals 

 the responses are rapid, and are carried out by contrac- 

 tions of protoplasm. The very simple animals, those 

 that do not have highly specialized tissues, react to such 

 features in the environment as light, temperature, sub- 

 stances in solution, gravity, and one or two others, in 

 definite fashions which demonstrate clearly their sensi- 

 tiveness. For example, the protozoan, Paramoecium, will 

 travel from highly illuminated regions to others less 

 brilliantly lighted. If a number are placed in a small 

 trough of water which at one end has a temperature of 

 25° C. (72° F.) and at the other a temperature either 

 higher or lower by several degrees, all the Paramoecia 

 will presently be found gathered at the end where the 

 temperature is 25° C. The adjustments thus made are 

 on the whole favorable to the organisms making them, 

 either through bringing them into the presence of abund- 

 ant food or by rendering them safe from harm. The 

 property of irritability is thus seen to serve a highly use- 

 ful purpose, even in the lowliest types. 



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