MASTIGOPHORA 41 



Owing to this fact it is impossible to frame a definition which 

 will enable every member of each subclass to be recognized as such 

 without comparison with other species. Certain characteristics, how- 

 ever, distinguish most members of the Zoomastigina from most of 

 the colourless Phytomastigina. These characteristics are stated below, 

 in the section which deals with the Zoomastigina. 



Subclass PHYTOMASTIGINA 

 Mastigophora which possess chromatophores, and species without 

 chromatophores which closely resemble such forms. 



There can be no doubt, for reasons which have been given above, 

 that this subclass contains the most primitive members of the phylum. 

 Its nutrition is extraordinarily interesting from that point of view. 

 Some of its species, notably among the Volvocina, are purely holo- 

 phytic. Others are normally also saprophytic, and some of these, like 

 Euglena, can upon occasion practise this mode of nutrition alone. Yet 

 others, like Polytoma, have become colourless, and are purely sapro- 

 phytic. Others again are both holophytic and, by amoeboid in- 

 gestion, holozoic. These lead insensibly to similar forms, members of 

 the Zoomastigina {Monas, etc.), which, being without chromato- 

 phores, have not the faculty of photosynthesis, but are purely animal 

 in their nutrition. Some of the coloured forms which possess a pit that 

 is called a gullet are said to take food with it, and thus to combine holo- 

 phytic and holozoic nutrition. In any case certain of their relatives 

 which have lost the chromatophores (Cyathomonas, Peranema, etc.) 

 take solid food through a similar gullet. Most of the holozoic forms 

 are probably also saprophytic. Certain species (Ochromonas, etc.) are 

 known to make use of all three modes of nutrition. Thus all ways of 

 obtaining nutriment meet in this group. 



The species which practise photosynthesis do so, like plants, by 

 means of chromatophores, of which they may possess one, two, or 

 many. The chromatophores are plate- or cup-shaped masses of proto- 

 plasm of a green, yellow, or brownish colour, owing to the presence 

 in various proportions of the pigments chlorophyll, xanthophyll, 

 carotin, etc. The chlorophyll absorbs the rays of sunlight whose 

 energy is used in photosynthesis. The green chromatophores are 

 known as chloroplasts, the yellow as xanthoplasts . Often there are to 

 be seen in or on the chloroplasts the protein bodies known aspyrenoids, 

 which act as centres of starch formation. A red pigment, haemato- 

 chrome, is frequently present diffused through the cytoplasm. In 

 bright light it spreads over the surface and is believed to shield the 

 chloroplasts from excess of certain rays. A small red spot of carotin, 

 sometimes darkened by another pigment, is generally present in 

 photosynthetic species, and probably acts as a rudimentary eye, 



