PROTOZOA. 323 



out from the body at a fixed point, the functional vent. 

 The large cilia always move in a regular and rhythmic man- 

 ner a fact which would imply that they were connected 

 and controlled in some manner in their action; and high 

 microscopic powers shows at their bases a cord of somewhat 

 denser material which takes the place of a central nervous 

 system. If this be cut, the cilia no longer work in har- 

 mony. Finally, all animals, in doing work, produce nitrog- 

 enous waste, which must be gotten rid of by means of 

 kidneys. In the form figured the kidneys are replaced by 

 a space on the interior (contractile vacuole) which regularly 

 enlarges and contracts, and at each contraction this waste 

 is forced out into the surrounding water. All of this is in 

 a single cell. 



The Protozoa, of which many thousand different kinds 

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

 being even visible without a microscope. The great 

 majority are aquatic, some being found in fresh water and 

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

 parasitic in other and higher animals, where they may at 

 times be productive of disease. 



