THE MASTIGOPHORA 157 



It is principally in respect of their modes of nutrition that the 

 Mastigophora appear to betray the mixed animal and vegetable 

 properties, so that as a class they have come to be regarded 

 mixotrophic micro-organisms. The four possible methods of ali- 

 mentation holozoic, parasitic, saprophytic, and holophytic are all 

 to be met with among the members of this protean series, either 

 separately or in combination. When a single species can vary its- 

 metabolism in adaptation to its immediate environment, for example, 

 according as it is exposed to or deprived of the influence of light, 

 it is said to be mixotrophic in the strict sense of the term (Pfefi'er). 



It is not always easy to assert positively in what manner food is- 

 conveyed into the protoplast (protoplasmic body of the cell), but it 

 is certain that holozoic nutrition is often associated in the same 

 species with saprophytic (Monadidea), saprophytic with parasitic, 

 saprophytic with holophytic (Euglenoidea), and, more rarely, 

 holozoic with holophytic (Chromulina). Sometimes three methods- 

 are found in combination holozoic, saprophytic, and holophytic 

 (Ochromonas). It may be stated as a general rule that all Lisso- 

 flagellata (i.e. true Flagellata in the restricted sense) are capable of 

 saprophytic nutrition, that is to say, of absorbing nutriment from 

 putrescent substances in an aqueous medium, but that this source 

 of food is usually accessory to some other essential means of 

 nourishment. Where saprophytism is the sole condition of exist- 

 ence, as in the case of the Astasiina, there is reason to regard it 

 as a secondary state derived, in the particular instance quoted, 

 from a condition of holophytism. 



The parasitic forms may be described broadly as falling into 

 three categories : ectoparasites (Costia, Stylochrysalis, Silicoflagellata) ; 

 endoparasites (species of Hexamitus, Megastoma, Tetramitus, Tricho- 

 mastix, Trichomonas, Trichonymphidae) ; and haematozoa (Trypano- 

 soma, Herpetomonas). 



The non-parasitic Mastigophora are either free -swimming or 

 sessile, solitary or colonial in habit. 



Some species are capable of temporary fixation by means of a- 

 protoplasmic stalk either of pseudopodial (e.g. Oicomonas sp., Fig. 5 

 (31)) or of flagellar (e.g. Bodo sp.) origin. Some solitary free forms- 

 are closely related to solitary fixed forms (e.g. Euglena and Ascoglena), 

 and many free-swimming colonial genera have sessile representatives 

 (e.g. Dinobryon and Hyalobryon). 



The form of association of individuals in the colonies varies 

 within limits, and there is a great amount of parallelism in this 

 respect between members of different orders. An entire colony or 

 coenobium may attain to a certain degree of individuation, which is 

 most marked in the Volvocina, but is not wanting in other groups, 

 as is evident from the co-ordinated movements which they execute 

 and from the fact that the whole coenobium may undergo binary 



