MASTIGOPHORA 47 



are probably also saprophytic. Certain species {OchromonaSy 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 as pyrenoids , 

 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, 

 making the organism sensitive to light, which is of such importance 

 in its nutrition. 



The holophytic forms are usually capable of passing into a resting 

 phase, in which the flagella are withdrawn, the body rounded off, a 

 cyst or jelly case secreted, and the organism closely resembles a plant 

 cell. Division may take place in that condition, establishing a pseudo- 

 colonial stage known as the palmella, and from this there may be 

 built up a branched body (Fig. 38 D, D^) which simulates those of the 

 lower algae. Plant-like forms of this kind occur in every order of the 

 group. It is indeed impossible to define the Phytomastigina from the 

 Algae, and the members of this subclass are regarded both by 

 botanists and by zoologists as coming within the scope of their 

 sciences. 



Many of the coloured species are liable to produce colourless In- 

 dividuals. This happens in two ways: the chromatophores may be- 

 come bleached owing to the animal living in darkness ; or the rate of 

 division of the chromatophores may lag behind that of the body, so 

 that eventually there are produced offspring in which there are no 

 chromatophores (" apoplastid " individuals). These facts show how 

 the colourless species may have arisen. 



Members of various orders of the Phytomastigina (cryptomonads, 

 a chrysomonad, a chlamydomonad, and perhaps dinofiagellates) are 

 known to live in the resting stage as symbionts in holozoic organisms 

 (other protozoa, sponges, coelenterates, worms, etc.). Nearly all are 

 yellow or brown (Zooxanthellae) ; most green symbionts {Zoochlorellae) 

 are algae belonging to the Protococcaceae. An exception to this is the 



