BIOLOGY OF THE PROTOZOA. 



CHAPTER I. 

 IXTRODUCTIOxN. 



A PROTOZOON is a minute animal organism, usually consisting of 

 a single cell, which reproduces its like by division, by budding, or 

 by spore formation and whose protoplasm has passed, or will pass, 

 through various phases of vitality collectively kno^\ii as the life 

 cycle. 



The maze of microscopic life to which the scientific world was 

 first introduced by Anton von Leeuwenhoek in 1675 included a 

 heterogeneous collection of animals and plants. Crustacea, rotifers, 

 minute worms, diatoms and desmids as well as the more minute 

 Protozoa, were all grouped together during the eighteenth and nine- 

 teenth centuries, first under the nondescript term animalculcB and 

 later under the more descriptive term Infusionsthiere of Ledenmiiller 

 (1763). The correct zoological position of the higher types of ani- 

 mals was recognized before the middle of the nineteenth century 

 and the group of strictly unicellular forms was first definitely out- 

 lined by von Siebold in 1S48 under the name Protozoa, a term sub- 

 stituted by Goldfuss (1820) for Oken's suggestive Urthiere (1805), 

 while the old name Infusoria has been retained for one of the sub- 

 divisions of the group. 



The haziness in classification of the older zoologists has not 

 entirely disappeared in the light of modern knowledge and we are 

 confronted today by the difficulties of distinguishing between 

 Bacteria, unicellular Alga^, and unicellular animals or Protozoa. 

 It is no reflection on modern science that we are unable to clearly 

 differentiate between these three groups. To accept the problem 

 as insoluble at the present time is merely to admit and apply our 

 conviction that evolution is now, and has been in the past, the pri- 

 mary biological principle underlying the diversities of forms and 

 functions of living things. Few biologists today will refuse to 

 accept the view that higher types of animals— Metazoa— have been 

 derived from forms in the past which were more or less similar to 

 present-day Protozoa; or the view that higher plants have been 

 evolved from unicellular plants. The variations and adaptations 

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