j CHAPTER V 

 Sensory Discrimination: the €hemical Sense 

 § 15. The Chemical Sense in Protozoa 



We have already seen that the most primitive type of 

 protozoon, Amceba 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 ciha 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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