88 ONE-CELLED ANIMALS— PHYLUM PROTOZOA 



quickly if placed in water that has been boiled and cooled unless time 

 is allowed for more oxygen to dissolve in it.) When oxygen combines 

 with the digested food in catabolism and converts the potential energy 

 into kinetic energy, three by-products result — carbon dioxide, ash, and 

 water. The carbon dioxide is eliminated by diffusion through the 

 plasma membrane, thus completing the process of respiration. The ash 

 consists of various minerals which formed a part of the protein food of 

 the amoeba — it is the so-called excretory waste. These minerals are in 

 solution and pass out of the cell through the plasma membrane in a 

 simple form of excretion. 



Water is a necessary part of protoplasm, but only so much water is 

 needed and any excess must be expelled. Water is produced in catabo- 

 lism and it is ingested along with the food. Also, an amoeba living in 

 fresh water will absorb water through its plasma membrane by osmosis. 

 These three sources provide more water than is needed ; if there were 

 not some way of letting off the excess, the amoeba would very likely 

 swell from this internal water pressure and might even burst. A con- 

 tractile vacuole forms, however, and eliminates the extra water. Under 

 the microscope this structure will appear as a spherical, clear body 

 within the cytoplasm and, as you watch, it will grow larger and then 

 suddenly disappear. The contractile vacuole originates near the center 

 of the cell and receives the excess water of the cell. It grows larger 

 as the water accumulates and migrates to the edge of the cell until, 

 finally, there is only a thin membrane separating it from the outside. 

 Then the membrane gives way and the water pours out. A new vacu- 

 ole will soon form and the process is repeated. If amoebae are placed 

 in water which has a concentration of dissolved minerals equal to or 

 exceeding that of the protoplasm, no contractile vacuoles are formed. 

 Some amoebae live in salt water, and these do not form contractile 

 vacuoles normally. When some of these are placed in fresh water they 

 will soon begin forming such vacuoles. These experiments establish 

 the water-regulating function of the contractile vacuoles. 



Under favorable conditions an amoeba will continue to grow until 

 it reaches a certain size and then it undergoes mitosis and forms two 

 cells of equal size. Fission is the term applied to such reproduction 

 where two organisms of equal size are formed. It is asexual reproduc- 

 tion since there is no mingling of nuclear materials from different cells. 

 So far as we know, the amoebae do not have any means of sexual repro- 

 duction. We can see from this description of reproduction that amoebae 

 never grow old and die of old age — they may live indefinitely, even 

 though split into many pieces. Actually, death is very frequent due to 

 destruction by other forms of life and to extreme changes in environ- 



