CHAPTER XI 



THE SIMPLEST, OR ONE-CELLED, ANIMALS 

 (PROTOZOA) 



Besides the Amceba, Paramwcium and Vorticella (de- 

 scribed in Chapter V) there are thousands of other kinds 

 of Protozoa. Most of them live in water, but a few live 

 in damp sand or moss, and some live inside the bodies of 

 other animals as parasites. Of those which live in water 

 some are marine, while others are found only in fresh-water 

 streams and lakes. 



Form of body. — The Protozoa all agree in having the 

 body composed for its whole lifetime of a single cell, * but 

 they differ much in shape and appearance. Some of them 

 are of the general shape and character of Amoeba, sending 

 out and retracting blunt, finger-like pseudopodia, the body- 

 mass itself having no fixed form or outline but constantly 

 changing. Others have the body of definite form, spherical, 

 elliptical, or flattened, enclosed by a thin cuticle, and having 

 a definite number of fine thread-like or hair-like protoplasmic 

 prolongations called flagella or cilia. Many of the familiar 

 Protozoa of the fresh- water ponds always have two whiplash- 

 like flagella projecting from one end of the body. By means 

 of the lashing of these flagella in the water the tiny creature 

 swims about. Others have many hundreds of fine short 

 cilia scattered, sometimes in regular rows, over the body- 

 surface. The Protozoan swims by the vibration of these 

 cilia in the water. 



There is no stagnant pool, no water standing exposed in 



*In some Protozoa a number of similar cells temporarily unite to form 

 a colony, but each cell may still be regarded as an individual animal. 



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