352 The Animal Kingdom 



of two gametes. In conjugation, there is temporary contact between 

 two individuals in which there is an exchange of nuclear material. 



Importance of the Protozoans. — Despite their tiny size, the pro- 

 tozoans are among the most important of the invertebrate phyla. Many 

 of the parasitic forms infest both man and his domestic animals. One 

 particularly, the organism causing malaria, has made whole sections 

 of the world unhealthy. The tremendous losses caused by the debility of 

 the individuals who are infected cannot be measured. 



Among the free-living forms, many species are the bases of food 

 chains necessary in the economy of fresh and salt water. These forms 

 eat bacteria or smaller protozoans and in turn furnish food for larger 

 forms. Possibly many of the soil-dwelling forms occupy a similar posi- 

 tion in soil cycles. 



One group, the Foraminifera, produce shells which have formed 

 the great chalk clififs of many sections of the world. Others of this 

 same group are important as indicators of oil-bearing strata. 



Biologically the protozoans have proved to be useful experimental 

 forms. Many are easily cultured in the laboratory. Due to their easy 

 availability, they have been studied extensively for information as to 

 how the individual cell functions. In recent years, they have been ex- 

 tensively utilized for the study of cytoplasmic inheritance. 



The Classes of Protozoans. — Five classes of protozoans are sep- 

 arated from one another on the basis of their mode of locomotion. 

 They are as follows : 



Class I. Mastigophora or Flagellata. These are the flagellates whose lo- 

 comotion is accomplished by means of a whiplike structure, the flagellum. Eu- 

 glena and Volvox are examples. 



Class II. Sarcodina or Rhizopoda. These move by means of pseudopodia. 

 Amoeba is the usual example of this class. 



Class III. Sporozoa. These forms are parasitic during all or most of their 

 life cycles. They have no specific locomotor organ during most of the life cycle. 

 Plasmodium, the causative organism of malaria, belongs to this class. 



Class IV. Ciliata. This is a very large group whose members move by 

 means of tiny hairs or cilia which cover the body surface. They were at one time 

 known as the Infusoria because of their abundance in hay infusions. Paramecium 

 is the most commonly studied form from this class. 



Class V. Suctoria. These are modified ciliates that have cilia only during 

 their young stages and are sessile when adult. Podophrya is in this class. 



