CHAPTER IX 



FUNCTIONAL ACTIVITIES OF PROTOZOA 



ALL living things are alike in that they perform certain fun- 

 damental activities. These, commonly known as the fun- 

 damental or vital functions, are : irritability, respiration, excre- 

 tion, nutrition, and reproduction * 



Irritability is the response of an organism to stimuli and may 

 be visible to the eye in the form of movement, or it may be molec- 

 ular and hence invisible. Respiration is the intake of oxygen and 

 the disposal of carbon dioxide. Excretion is the process of rid- 

 ding the body of poisonous waste products such as carbon diox- 

 ide, urea, etc. Nutrition is the aggregate of activities having to 

 do with obtaining food, digesting and assimilating it, and adding 

 it to the protoplasmic substances already present. Reproduction 

 is the means by which a life-form is continued and multiplied. 

 In the long run cell division is the only method of reproduction. 

 This is plainly evident in these primitive forms, but is more ob- 

 scure in the metazoa (many-celled animals), where phenomena 

 of fertilization are necessary to ensure development. 



Each function is an aggregate of activities, some of which 

 can be controlled and measured, but not in equal degree. Nutri- 

 tion, for example, may be controlled through the food supply, 

 and various stages of inanition+ brought about by starvation 

 may be studied. All of these functions operate at the same time, 

 and failure of any one link in the chain of activities constituting 

 these functions leads to death and disintegration. Reproduction 

 of protozoa has been considered in the preceding chapter. 



Irritability 



Irritability visibly manifests itself by various kinds of move- 

 ment that are determined by the derived organization. The 

 energy necessary to produce this movement is obtained through 



* See "The Coming and Evolution of Life," page 7, in this Scries. 

 t Inanition, exhaustion from lack of food. 



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