THE ORGANIZATION OF THE PROTOZOA 63 



ingested as food are suspended and gradually digested. In some 

 cases, however, and especially when the prey is relatively large, 

 no distinct fluid vacuole can be made out surrounding it, but the 

 food appears to be simply lodged in the endoplasm itself ; the 

 vacuole is " virtual." When the digestion is completed, the in- 

 soluble faecal residues are cast out of the body. 



In Protozoa in which the body consists of naked, non-corticate 

 protoplasm, the food is ingested, and the faecal remains are expelled, 

 at any point on the surface of the body. In corticate Protozoa, 

 on the other hand, in which the body is limited by a resistant 

 envelope or cuticle of a certain strength and thickness, food can- 

 not be ingested at any point, but is taken in through a special 

 aperture, a cell-mouth or cytostome. In such cases the organs of 

 food-capture are either flagella or cilia, and by their action the food 

 is wafted into the mouth. Primitively the mouth is a superficial 

 aperture in the cuticle, opening into the endoplasm by means of a 

 longer or shorter tube, the oesophagus or cytopharynx. In the 

 Peritricha (p. 433), however, the mouth and oesophagus are, as it 

 were, carried into the body at the end of an in-sinking of the ecto- 

 plasm, which forms a Jong tube or vestibule, comparable in its 

 mode of formation to the stomodaeum of the Metazoa. In any case 

 the food-vacuoles are formed at the bottom of the oesophagus, in 

 the endoplasm. The mode in which the vacuoles arise, and the 

 processes of digestion and defaecation, are discussed in a subsequent 

 chapter (p. 189, infra). 



2. In holophytic forms assimilation is carried on by cell-organs 

 of the same nature as those found in the green cells of ordinary 

 plants. Of primary importance are the chromatophores, or chromo- 

 plasts, bodies containing chlorophyll or allied pigments by means of 

 which the organism is enabled to decompose carbon dioxide in the 

 sunlight, setting free the oxygen and utilizing the carbon for build- 

 ing up the living substance. The chromatophores vary greatly 

 as regards size, form, and number present in the cell-body. Other 

 bodies of constant occurrence are pyrenoids, small glistening cor- 

 puscles which appear to serve as centres for the formation or storage 

 of starch or similar substances of amyloid nature produced in the 

 process of anabolism (see infra, p. 188). 



In any Protozoa, whatever their mode of nutrition, the endo- 

 plasm contains usually various enclosures, which can be classed 

 generally as metaplastic that is to say, as products of the upward 

 (anabolic) or downward (catabolic) metabolism of the living sub- 

 stance. Instances of anabolic products are the grains of starch or of 

 the allied substance, paramylum, found in the holophytic forms, 

 and the reserve food-materials fat, " paraglycogen," and other 

 substances often stored up in considerable quantity in prepara- 



