CHAPTER XVII 



PROTOZOA IN GENERAL 



The phylum Protozoa (pro to zo' a; G., protos, first, and zoon, animal)' 

 includes all one-celled animals, the one cell which forms the body of the 

 individual carrying on in simple fashion all of the general functions which 

 are performed by the many-celled bodies of higher animals. This means 

 that though the animals included in Protozoa are simple in that they are 

 composed of only one cell, this cell is physiologically complex. Some of 

 the Protozoa always exist as single cells. Others are associated in colonies 

 in which they are all alike and each quite independent. In other proto- 

 zoan colonies, however, certain functions, such as reproduction, are 

 assumed by certain cells, which thus become reproductive individuals. 



106. Classification. — Protozoa is the first phylum of the animal 

 kingdom, but, since all other phyla have characteristics which they share 

 and which distinguish them from Protozoa, it may be considered also 

 as a group of higher rank than a phylum. In this case it becomes a 

 subkingdom, with the same name, coordinate with the subkingdom 

 Metazoa, which includes all the remaining phyla. 



Protozoa may be characterized as composed of animals existing as 

 single cells. In the case of certain types these one-celled individuals are 

 associated in colonies. 



The phylum is usually divided into four classes, each characterized 

 by a distinctive locomotor structure or, in one class, by the absence of 

 any such structure in the final stage of the animal. These classes are: 



1. Mastigophora (mas ti gof o ra; G., niastix, whip, and phoros, 

 bearing), or Flagellata (fla gel la' ta). — Have a limited number of long 

 whiplike locomotor appendages known as flagella. 



2. Sarcodina (sar ko di' na; G., sarkodes, fleshy), or Rhizopoda 

 (rl zop' o da; G., i^hiza, root, and podos, foot). — Form pseudopodia, 

 which are temporary structures developed from the surface of the body 

 and which can be withdrawn. 



3. Sporozoa (spo ro zo' a; G., spoi^a, seed, and zoo7i, animal). — Possess 

 no locomotor structures in the final stage, though they have them in the 

 earlier stages of their life histories. 



1 The vowel sounds indicated in the pronunciation of this and other phjda and 

 class names are described at the beginning of the Glossary (p. 557). In all cases 

 where the nominative form of a Latin or Greek word does not contain the full root, 

 the genitive is giv(>n, as, for instance, podoa, genitive, instead of pons, nominative, 

 for the Greek word for foot. If a word comes from the Greek through the Latin, 

 the Greek is given. 



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