PROTOZOA. 147 



are represented by a space on the interior (contractile vacu- 

 ole) which regularly enlarges and contracts, and at each 

 contraction this waste is forced out into the surround- 

 ing water. All of this is in a single cell. 



Occasionally there occur compound Protozoa in which 

 several, or even hundreds, of cells (animals) may be united 

 in a 'colony,' but even in these cases the essential fea- 



/ / 



tures persist. There is no differentiation among the cells, 

 each being like all the rest and each for itself carrying 

 on all the functions of life. 



The Protozoa, of which many thousand different kinds 

 have been described, are very minute, only a few being 

 visible without a microscope. The great majority of 

 them are aquatic, and they are abundant in stagnant 

 water, especially that which contains much decaying 

 animal or vegetable matter. Some occur in fresh water, 

 many in the sea. A few live in moist earth and more are 

 parasitic in other and higher animals, w r here they may 

 be productive of disease. 



As they are the simplest of animals, so they are regarded 

 by naturalists as the oldest, it being believed that at one 

 time that they, together with the simplest plants, were 

 the only forms of life upon the earth. 



The Protozoa are divided into three classes : Rhizopoda, 

 Infusoria, and Sporozoa. 



CLASS I. RHIZOPODA. 



These are the least specialized of the Protozoa and in 

 them cell organs are few. There is no denser external layer 

 to the body and hence they are able to thrust out lobes 

 or pseudopodia from any part of the surface and to retract 

 them at will. By means of these pseudopodia the animals 



