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L I 8 R A R 

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AN INTRODUCTION TO THE STUDY 

 OF THE PROTOZOA 



CHAPTER I 



INTRODUCTORY THE DISTINCTIVE CHARACTERS OF THE 

 PROTOZOA AND OF THEIR PRINCIPAL SUBDIVISIONS 



THE Protozoa are a very large and important group of organisms, 

 for the most part of minute size, which exhibit a wide range of 

 variation in structural and developmental characters, correlated 

 with the utmost diversity in their modes of life. Nevertheless, 

 however greatly adaptation to the conditions of life may have 

 modified their form, structure, or physiological properties, a certain 

 type of organization is common to all members of the group. The 

 most salient feature of the Protozoa is their unicellular nature ; 

 that is to say, the individual in this subdivision of living beings is 

 an organism of primitive character, in which the whole body has 

 the morphological value of a single " cell," a mass of protoplasm 

 containing nuclear substance (chromatin) concentrated into one or 

 more nuclei. However complex the structure and functions of the 

 body, the organs that it possesses are parts of a cell (" organellae "), 

 and are never made up of distinct cells ; and at least one nucleus 

 is present, or only temporarily absent, as a constant integral part 

 of the organism. The unicellular nature of the Protozoa, though a 

 constant character, cannot, however, be used by itself to define 

 the group, since it is also a peculiarity of many other distinct types 

 of simple living things. 



As an assemblage of organisms of primitive nature from which, 

 in all probability, the ordinary plants and animals have originated 

 in the remote past by divergent processes of evolution, the Protozoa 

 have always possessed very great interest from the purely scientific 

 and philosophical point of view. Of recent years, however, they 

 have also acquired great practical importance from the relations 

 that have been discovered to exist between Protozoa of parasitic 

 habit and many diseases of man and animals. Hence the study of 

 the Protozoa has received an immense impetus, and has been 



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