CHAPTER IV 



PHYLUM PEOTOZOA IN GENERAL 



The animals included in this group are usually said to be the 

 first to have existed on earth and, therefore, they are considered 

 the oldest. Being single-celled, they are usually referred to as the 

 simplest known animals, although many of them are perhaps more 

 complicated than numerous many-celled or metazoan forms because 

 of the extensive modifications of the one cell. Protozoa are univer- 

 sally placed first when animal groups are placed in the order of 

 complexity, beginning with the simplest. It has been supposed and 

 with reasons to support the supposition, that modern Protozoa have 

 descended, without changing their single-celled condition, from 

 primitive organisms that were also the ancestors of Metazoa. 



Characteristics 



The great majority of Protozoa are microscopic creatures. Most 

 of them live in water while a few live in the body fluids of other 

 animals. Certain types are found living rather abundantly in the 

 soil water. They are found in almost all conceivable shapes. Some 

 have irregular, changing shapes ; others are nearly spherical, oval, 

 spindle-shaped, cylindrical, and vase-shaped. Most Protozoa exist 

 singly as an independent cell, but some are organized into groups 

 called colonies. A few are encased in hard coverings or shells which 

 are made up of a secretion from the cell alone, or of a combination of 

 such a secretion with a foreign material like sand. "With the excep- 

 tion of one class the Protozoa have characteristic locomotor organs. 



^to' 



Classification 



This group is often spoken of as a subkingdom as well as the first 

 phylum of the animal kingdom. In spite of the exceedingly large 

 number of species and microscopic size, the phylum has been quite 

 systematically classified and is divided into classes, orders, families, 

 genera, and species. The phylum is usually divided into four classes, 

 each characterized by a distinctive locomotor structure or by the 

 total lack of such features, as in one of the classes. 



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