PHYLUM PROTOZOA. ONE-CELLED ANIMALS 



31 



behavior, and habitat, will be studied in de- 

 tail later in this chapter, 



CHARACTERISTICS OF 

 THE PROTOZOA 



One who examines a bit of pond scum 

 under the microscope for the first time feels 

 as though he were discovering a new world. 

 The protozoans that become visible as a 

 result of magnification do not come within 

 our everyday experience because they are 

 microscopic in size. If enormous numbers of 

 them are crowded together, they may im- 

 part their color to the water in which they 

 live, as the green species Euglena some- 

 times does to a fresh-water pond. However, 

 few species are large enough to be seen with 

 the naked eye when only one specimen is 

 present. 



Active protozoans are unable to live where 

 it is dry, but they are abundant almost every- 

 where in water or in moist places. Fresh- 

 water ponds, lakes, and streams abound in 

 them; billions live in the sea; the soil often 

 teemxS with them to a depth of several inches 

 where it is moist; and large numbers live on 

 the outside or within the bodies of other 

 animals. 



Most protozoans are unicellular, that is, 

 they consist of a single cell; but a few con- 

 sist of groups of cells. If they are composed 

 of a group of cells, the cells are not differen- 

 tiated into tissues. The Protozoa are the 

 most primitive of the large groups of ani- 

 mals and stand in contrast to most of the 

 others, which are many-celled tissue animals. 

 It seems quite remarkable that such minute 

 organisms are capable of maintaining them- 

 selves in a world inhabited by so many larger 

 and more complex animals. 



In spite of the small size and vast num- 

 bers of species of Protozoa, it is not difficult 

 to arrange them in classes, orders, families, 

 genera, and species. The Protozoa are di- 

 vided into 4 classes on the basis of the struc- 

 ture they possess for locomotion. One exam- 



ple from each class is described in the fol- 

 lowing chapters. The 4 classes of Protozoa 

 are as follows: 



Class 1. Sarcodina. Type: Amoeba proteus. 



Protozoa that move by means of 



false feet called pseudopodia. 

 Class 2. Mastigophora. Type: Euglena viridis. 



Protozoa that move by means of 



whiplike processes called flagclla. 

 Class 3. Sporozoa. Type: Monocystis lum- 



brici. 



Protozoa without motile organelles, 



but with a spore stage in their life 



cycle. 

 Class 4. Ciliata. Type: Paramecium cauda- 



tum. 



Protozoa that move by means of cilia. 



AMOEBA PROTEUS 



Habitat and preparation 

 for study 



From "amoeba to man" is a common ex- 

 pression often seen in the popular press, sug- 

 gesting that all living animals are found be- 

 tween these extremes, with the amoeba 

 representing the lowest form of life and 

 man the highest. Whether or not this ex- 

 pression is true is open to question, and 

 after you have made a comprehensive study 

 of animal life you will understand why this 

 is said. Amoebas live in many different habi- 

 tats, such as fresh water, the sea, the soil, 

 and as parasites within other animals, in- 

 cluding man. A common large fresh-water 

 species, and one that is usually selected to 

 introduce the phylum Protozoa, is Amoeba 

 proteus (Gr. amoibe, change; Proteus, a sea 

 god in classical mythology who had the 

 power of changing his shape). The amoeba 

 (Fig. 11) lives in fresh-water ponds and 

 streams. It can often be found on the under- 

 side of dead lily pads and other vegetation 

 in shallow water. 



If material containing amoebas is studied 

 under a microscope, some of the activities 



