THE MODES OF LIFE OF THE PROTOZOA 15 



the Flagellata, where a given species may be either holozoic or 

 holophytic,* according to circumstances. 



The bionomics of Protozoa that is to say, their relations to their 

 environment and to other organisms constitute a very important 

 branch of knowledge, both practical and theoretical, especially in 

 the case of parasitic forms. Considering the subject from the point 

 of view of the four modes of life already described, it is clear, in 

 the first place, that the holophytic forms are entirely independent 

 of all other living organisms, and require for their continued 

 existence only sunlight and a suitable environment, containing the 

 necessary inorganic substances, at a temperature which permits 

 the continuance of vital processes and activities. 



Saprophytic organisms, however, in so far as they require for 

 their sustenance materials produced by living bodies, are dependent 

 directly or indirectly upon other organisms for their existence. 

 Purely holozoic forms, also, cannot exist without other forms of 

 life upon which, or upon the products of which, they can feed. 

 But neither holozoic nor saprophytic organisms are dependent, 

 as a rule, upon any other particular form of life, but only upon living 

 things generally ; though in some cases such forms may be specialized 

 in their nutrition to such an extent as to be unable to exist without 

 some particular food. 



A parasitic form, on the other hand, is entirely dependent, as a 

 rule, for its existence on some particular organism or limited group 

 of organisms which constitute its host or hosts. It must, however, 

 be understood clearly that an organism living in or upon the body 

 of another organism is not necessarily a parasite by any means. 

 In the first place, a distinction must be drawn between parasitism 

 and symbiosis, by which is meant an association of two organisms 

 for mutual benefit. f Good examples of symbiosis are seen in 

 some of the Sarcodina, Radiolaria, and Foraminifera, the proto- 

 plasm of which contains constantly intrusive organisms, known as 

 zoochlorellce or zooxanthdlce, according as they contain a green or a 

 yellow pigment. Zoochlorellse are green algae of the order Proto- 

 coccacece ; zooxanthellse are holophytic flagellates of the suborder 

 Cryptomonadina e.g., Cryptomonas schaudinni, symbiotic in the 

 foraminifer Peneroplis (Winter, 28). These organisms penetrate 



* For example, the species Chromulina flavicans. See Biitschli (2), vol. ii., p. 865. 



t The term " symbiosis " is often much misused, especially by medical writers, 

 by whom it is commonly applied to any association of two distinct organisms ; 

 for instance, "pure mixed cultures" of amoebae with some species of bacillus, 

 where the amoebae are simply feeding on the bacteria, are often spoken of as 

 '' symbiosis," although the advantage is clearly only on one side in such an asso- 

 ciation. It should be understood that the term " symbiosis " is a technical term 

 of long standing in biology, and is used not merely in its strict etymological sig- 

 nificance of " living together," but in the special and restricted sense of " living 

 together for the mutual benefit of the two organisms concerned." 



