CHAPTER XIV 



PROTOZOA 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 universally placed first 

 when animal groups are placed in the order of complexity, begin- 

 ning with the simplest. It has been supposed, and with reasons to 

 support the supposition, that modern Protozoa have descended, with- 

 out changing their single-celled candition, 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 exception 

 of one class the Protozoa have characteristic locomotor organs. 



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. 



1. Class Mastig-ophora (mas ti gof 6 ra), Avhich means whip bear- 

 ers, includes forms that possess one or more whiplike extensions of 

 the cytoplasm, or flagella. The number of flagella is limited, and 



268 



