INTRODUCTION 



for example, among the flat worms the Tiirbellaria are regarded as 

 the most primitive class in the phylum Plathelminthes, while the 

 Trematoda are considered as a higher class. This does not mean 

 that trematodes had their origin from the turbellarians but prob- 

 ably signifies that both trematodes and turbellarians had a com- 

 mon origin through some ancestral group of past time which was 

 neither trematode nor turbellarian. 



Thus, instead of a linear arrangement illustrating the modern 

 idea of phylogenetic relationship of the animal groups, this 

 relationship is best expressed as a branching treelike structure 

 (Fig. 1) in which the the phylum protozoa 



various groups are repre- 

 sented by the branches. 

 Closely related groups 

 would be represented by 

 a forking of a common 

 ancestral branch. One 

 point wherein the tree 

 comparison may not be 

 carried too far is that 

 many of the branches 

 connecting modern-day 

 forms are dead or extinct. 

 Through the field of 

 paleontology we have an 

 imperfect picture of these 

 connecting branches in 

 the records left as fossil 

 animals — imperfect be- 

 cause there are so many 

 groups like the naked 

 Protozoa and the worms 

 of which but few fossils ^^^- ^^ 

 are known. Because of 

 the significance of these extinct animals in considering our modern 

 fauna, attention is directed to some of the more important fossil 

 forms throughout the body of this book. The necessity of 

 considering our fauna as a product of the animal life of the past 

 makes it seem advisable to include here a reference table show- 

 ing the sequence of some of the more important geological periods 

 with some of the dominant forms of life characteristic of each. 



Di'&iincfl^ Animal 



jj Plani--Lil^ 



I 



5i 



Phylogenetic arrangement of the classes 

 in the phylum Protozoa. 



