CHAPTER 8 



The Phylum Protozoa 



46. Introduction 



The single-celled animals remained unknown until Antony van 

 Leeuwenhoek, a Dutch lens-maker oi the 17th century, examined water 

 droplets with a primitive microscope and discovered that diverse, very 

 small lorms ol lite existed. His work received international attention 

 and his organisms were studied extensively. At that time, however, the 

 cellular nature ot organisms was not known and van Leeuwenhoek's 

 "little animals" were considered by most investigators to be merely small 

 varieties of worms or wormlike animals. It was not until 1845 that the 

 unicellular nature oi many ot these microscopic animals was appreciated, 

 and the phylum Protozoa was established to include them. Today this 

 phylum includes all ot the one-celled animals. Since all other animals 

 are multicellular, the animal kingdom is otten divided into the subking- 

 dom Protozoa, including only the phylum Protozoa, and the subkingdom 

 Metazoa, including all the other phyla. 



The typical protozoan has a single nucleus and leads an independent 

 existence. In some, however, the cell is multinuclear, while in others the 

 individual cells are attached and form colonies. Most of the individuals 

 in a protozoan population are produced by simple cell division of the 

 parent, although sexual reproduction is by no means rare. 



Protozoa are primarily aquatic, living in bodies of water of all kinds, 

 fresh and salt, from puddles to oceans. Some live in damp soils, crawling 

 in the thin film of water surrounding dirt particles. Others are parasitic 

 and live in the fluids of animals and plants. Whatever their habitats, 

 the surfaces of active protozoa must remain wet, for they cannot survive 

 desiccation. 



Variation in form is enormous. Some protozoans are shapeless 

 "blobs" while others are as elaborate and as geometrically patterned as 

 snowHakes. In some groups the cells may have internal skeletons, external 

 skeletons, or protective houses cemented together from sand and other 

 particles. Those with hard parts can be fossilized. Only two thirds of the 

 25,000 described species of protozoa are living. The others are known 

 from their remains found in rocks. 



The phylum is divided into five classes (Fig. 8.1): (1) Flagellata, the 

 flagellates, having one or more long, whiplike flagella; (2) Sarcodina, in 

 which pseudopods are formed for locomotion and feeding; (3) Ciliata, 

 the ciliates, characterized by the presence of many short cilia for locomo- 



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