CHAPTER V 



SENSORY DISCRIMINATION: THE CHEMICAL SENSE 

 15. The Chemical Sense in Protozoa 



WE have already seen that the most primitive type of 

 protozoon, Amoeba proteus, discriminates between edible 

 and inedible substances. While it will sometimes ' swal- 

 low' inedible particles such as grains of carmine, it takes 

 immediate measures to get rid of them, measures too prompt 

 to be the result of an actual attempt at digestion, and 

 hence properly to be regarded as the effect of a chemical 

 or food sense. Many other members of the lowest division 

 of the animal kingdom, the Protozoa, have a structure and 

 behavior decidedly more complicated than those of Amoeba. 

 There is a large group of single-celled animals called Ciliata, 

 from the fact that their bodies are covered with little hair- 

 like protoplasmic filaments or cilia which serve as organs 

 of locomotion by acting like tiny oars. A common repre- 

 sentative of the group is Paramecium. The structure of 

 this animal is distinctly more specialized than that of 

 Amoeba. Not only are the cilia modified locomotory 

 structures, but there is a definite region for food-taking. 

 A groove extends obliquely down one side of the body, 

 terminating at its lower end in a mouth. The cilia along 

 this oral groove beat with especial vigor and create currents 

 which sweep food particles to the mouth. Paramecium 

 swims rapidly through the water with a spiral motion of 

 its body, due to the facts that the aboral cilia beat more 



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