PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 83 



The free-living forms of protozoa are almost constantly at work; 

 they are usually in motion, either in progressive movement, or, by 

 action of their flagella or cilia, are creating currents toward the 

 mouth. The energy for such work comes from the breaking down of 

 complex molecules of protoplasm or possibly of digested food, which 

 is accomplished by oxidation or physiological burning. The products 

 of such combustion, as in physical combustion, are kinetic energy, 

 heat, and residual matter, and the latter, like ashes, must be disposed 

 of, or bv accumulation thev hinder and ultimatelv prevent the normal 

 processes. The ordinary products of such physiological activity are 

 solid or fluid matters consisting mainly of water, some mineral sub- 

 stances, urea, and a gas, carbon dioxide. In higher animals the former 

 are disposed of through the medium of the skin in part, but mainly 

 through the activity of the kidney, while the latter are thrown out 

 through the skin and lungs, or gills. In protozoa, while there is the 

 same need of elimination of the waste materials, there is in many 

 forms no especial organ for the purpose, elimination of urea and of 

 carbon dioxide taking place, as in some intestinal parasitic worms, by 

 osmosis through the general surface of the bodv. Such is the case in 

 all of the foraminifera and radiolaria, and in individual cases among 

 the other types of protozoa. In other forms of protozoa, however, 

 there may be special organs for the disposal of such waste matters. 

 These are the contractile vacuoles which fill with fluids from the 

 interior of the cell and then contract, emptying their contents to the 

 outside through a minute pore, as in the majority of infusoria, or 

 breaking through the outer wall of protoplasm at any point where the 

 vacuole may be at the time of contraction, as in amebea. The fluids 

 of these contractile vacuoles are supposed to hold urea in solution as 

 well as carbon dioxide, the experiments of Griffiths ('89) indicating 

 the presence of urea, while biologists generally agree that carbon 

 dioxide must also be present in the fluids discharged, although in no 

 case has this been proved. Another function of the contractile vacuole 

 may be, as Hartog early pointed out, the regulation of the tension in 

 protoplasm and surrounding water and the prevention of large dis- 

 ruptive vacuoles through the constant addition of water taken in In- 

 the crystalloids of the cell. Whatever may be the function of the 

 vacuole, it becomes a very important element of the cell in the more 

 complicated forms of protozoa, and is frequently associated with long, 

 branching feeding canals, which to Ehrenberg were evidences of a vas- 

 cular system, since they ramify through the protoplasm, collecting fluid 

 which is (Mnj)tied into the contractile vacuole. While the function of 

 such contractile vacuoles is elimination of waste or regulation of 

 density, they cannot be absolutely necessary to protozoa, nor the sole 

 means of disposing of waste materials, since great numbers of protozoa 

 are without them. 



