CHAPTER II 

 THE MODES OF LIFE OF THE PROTOZOA 



PROTOZOA, as simple protoplasmic organisms, can only exist in an 

 active state in a fluid medium. Hence the free-living, non-parasitic 

 forms are aquatic, either marine or fresh-water in habitat. A 

 certain number of species, however, are semi-terrestrial in their mode 

 of life, creeping on damp surfaces or living in a minimum of 

 moisture. Examples of such forms are the Amoebae, etc., found in 

 the soil, or Mycetozoa, which in the plasmodial phase (p. 239) creep 

 on tree-trunks, logs, and so forth. None of these forms, however, 

 can remain active in perfectly dry surroundings, but pass into a 

 resting state when desiccated. 



It has been stated already that the methods by which Protozoa 

 gain their livelihood vary greatly in different cases. Considered 

 generally, these methods may be classified under four types : 



I. The majority of Protozoa nourish themselves after the manner 

 of animals that is to say, they are entirely dependent for food and 

 sustenance on other organisms which they capture, devour, and 

 digest. Such forms are said technically to be holozoic, a word sig- 

 nifying " completely animal-like "; they are unable to utilize simpler 

 chemical substances in order to build up the protein constituents 

 of the living body, but require proteins ready-made for their 

 sustenance. 



II. A certain number of Protozoa all, with rare exceptions, 

 belonging to the class Mastigophora possess in their body-sub- 

 stance peculiar colour-bearing corpuscles, so-called chromatophores or 

 chromoplasts, containing chlorophyll or a pigment of allied nature, 

 by means of which they are able to decompose carbon dioxide in 

 the sunlight, liberating the oxygen and making use of the carbon 

 in order to build up the protein and other constituents of the living 

 body. Such organisms are entirely similar in their metabolism 

 to the ordinary green plants, and are hence termed holopliytic. or 

 " completely plant-like." 



The holophytic condition, in which the chlorophyll-bodies form an integral 

 part of the structure of the body, and are to be regarded simply as proto- 

 plasmic organs, must be distinguished carefully from a state of things often 



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