THE SIMPLEST ANIMALS 305 



No Circulation. Especially is it noteworthy that, unlike 

 the frog and other higher animals, a paramecium carries 

 on its life-activities without organs for circulation (heart and 

 blood-vessels) . Turn back and review the reasons why a frog 

 needs these organs ( 52), and it will be evident that none of 

 the reasons given applies to a paramecium, for that animal 

 is so small that digested food, oxygen, and excretions do not 

 require transportation to and from distant parts of the body. 

 It is only a short distance from the surface of the body to the 

 innermost particles, and so oxygen and carbon dioxide can 

 diffuse as easily as they go through the wall of a blood- 

 capillary in a frog. Also, the movement of the inner layer 

 of protoplasm in a paramecium helps distribute food while 

 it is being digested. 



Irritability. A paramecium shows no nerves or other 

 structures which appear to be substitutes for the nervous 

 system in a frog ; nevertheless the animal responds to stimuli 

 caused by chemicals, touch, light, and electricity. It has 

 no nervous organs, but it has irritability (response to stimuli) 

 in a simple form. 



Motion. A paramecium has movement, and yet there 

 are no visible muscles. Instead, the protoplasm of its entire 

 body seems to have the power of movement or contractility. 

 This is especially developed in the protoplasm composing the 

 cilia. 



Reproduction. And finally a paramecium has the power 

 of reproducing, without which its species would soon cease 

 to exist. Since it consists of a single cell, its only possible 

 way of producing new individuals is by cell-division a simple 

 process of asexual reproduction. And to provide for fre- 

 quent mixing of protoplasm from two individuals, which is 

 of great significance in heredity, Paramecium also has a 

 simple process of fertilization or conjugation. 



Summarizing, we find within a single cell of this simple 

 animal all the life-processes which we find in higher animals; 



