CHAPTER XLIII 



TROPISMS AND THE BEGINNINGS OF SENSE 



261. Tropisms. In the absence of neurons in the simplest 

 animals we cannot speak of their reflexes. Most of the reac- 

 tions that have been studied are classed 

 as tropisms. Tropisms have been ex- 

 plained as resulting from the unequal 

 contraction of the protoplasm on oppo- 

 site sides of the body, under the influence 

 of unequal, or one-sided, stimulation. 



262. The general reaction of lower 

 animals. Many organisms are not sym- 

 metrical, and many, like the Paramecium, 

 or slipper animalcule, make progress in 

 a given direction by moving spirally 

 around the line that represents this 

 direction (see Fig. 87). In response to 

 any disturbance or change in condition, 

 such animals always make the same 

 movement (see Fig. 88). 



As a result of this "general reaction" 

 to all kinds of stimuli, the animal man- 

 ages to escape many dangers, and to 

 get into situations that are frequently 

 advantageous (see Fig. 89). 



263. Chemical sense in lowest organ- 

 isms. The simplest animals, like the 

 roots of many plants, are sensitive to 



many kinds of chemical disturbance. We cannot suppose that 

 an ameba, for example, has the feeling of sour or sweet, or 



224 



FIG. 87. Movement in 

 Paramecium 



In the Paramecium, as in 

 many other free-swimming 

 protozoa (one-celled animals), 

 the organism moves forward 

 by spinning about its own 

 axis and at the same time 

 swinging in a spiral path 



