164 INTRODUCTION TO ZOOLOGY. 



A table of resemblances among the vorticella, 

 stentoi*, paramcEcium, and amoeba will show the fol- 

 lowing : — 



each consists of a single animal cell 



composed of protoplasm 



ingest food in solid particles 

 Vorticella \ , ^,-, » 



I exchange CO2 for oxygen 

 Stentor / 



_ . < aquatic 



Paramoecium \ , , 



, eaon has a nucleus 



Amoeba I 



are very small 



exhibit self activity 



multiply by fission 



This gives us a concept of a Protozoan. A Pro- 

 tozoan is an animal that consists of a single animal 

 cell, is composed of protoplasm, etc. 



The Protozoa constitute a very large and important 

 branch of animals. In number of individuals at any 

 one time, they probably exceed all other animals 

 taken together. In degree of complexity, there is 

 perhaps as much difference between the simplest Pro- 

 tozoan and the most complex Protozoan as there is 

 between the simplest and the most complex animals 

 of any other branch. 



It is very interesting to observe the change that 

 has taken place in the ideas of men concerning ani- 

 mals. The first division of the animal kingdom was 

 into two groups, the Vertebrates and the Invertebrates. 

 But it was early discovered that among the Inverte- 

 brates were several groups entitled to equal rank with 

 the Vertebrates, so a division into four groups was 

 recognized : Vertebrates, Articulates, Radiates, and 

 MoUusks. Still further study showed that Articulates 

 should be separated into Arthropods and Vermes. 

 Tlien the sponges were recognized as animals, and 

 the Protozoa were discovered ; also, the Coelenterata 

 were separated from the Radiates. 



