14 THE PROTOZOA 



found in holozoic Protozoa of all classes namely, the presence in the body 

 substance of symbiotic independent organisms of vegetable nature, as described 

 below. 



III. A certain number of Protozoa that have no chlorophyll or 

 similar pigment in their bodies are, nevertheless, free from the 

 necessity of preying upon other organisms in order to obtain their 

 sustenance, since they are able to live upon organic substances in 

 solution, the products of the metabolism or decay of other living 

 organisms. Such forms are termed saprophytic (or saprozoic), since 

 their mode of life is similar to that of a saprophyte, such as a fungus. 

 It is not necessary that they should be supplied with ready-made 

 proteins in their food, since they are able to build up their protein 

 constituents from substances of simpler chemical nature. Many 

 examples of saprophytic forms are found amongst the free-living 

 Flagellata. 



Lauterborn (17) has coined the useful term sapropelic (from the Greek 

 frr)\6s, mud) to denote a mode of life which must be regarded as a special 

 type of the saprophytic method, partly also of the holozoic namely, the 

 mode of life of those fresh- water organisms that live in a mud or ooze composed 

 almost entirely of the decaying remains of dead plants and other debris of 

 a similar nature. A very characteristic fauna occurs under these conditions. 



IV. Finally, many Protozoa of all classes live as parasites that 

 is to say, at the expense of some other living being, which is termed 

 the host. 



These four modes of life can be used only to a very limited extent 

 for classificatory purposes ; it is only possible to do so in those 

 cases where a particular habit of life, long' continued, has resulted 

 in definite structural characteristics, and more especially in the 

 loss of organs requisite for other modes of life as, for example, in 

 the case of the subdivision Phytoflagellata, of the order Flagellata, 

 where the holophytic habit has become so ingrained that only 

 structural features proper to vegetable life are retained. 



In other cases it is clear that a given habit of life in different 

 organisms does not necessarily indicate close affinity between them. 

 In the first place, we find closely allied forms living in different 

 ways. Examples of all the four methods of metabolism described 

 above are to be found in the single order Flagellata, and through- 

 out the Protozoa there are commonly to be found parasitic forms 

 closely allied to free-living forms. In the second place, different 

 types of metabolism may be found as transitory phases in the life 

 of one and the same individual or species. Thus the common 

 Euglena (Fig. 4), a flagellate possessing chromatophores and living 

 normally in a holophytic manner, is able to maintain itself as a 

 saprophyte if deprived of the sunlight necessary for a holophytic 

 mode of life (p. 188) Striking examples of variability in the 

 mode of nutrition are seen also in the section Chrysomoiiadina of 



