General Morphology of the Protozoa 33 



induced by reduction in oxygen tension of the medium, causes the organ- 

 ism to rise toward the surface. Perhaps the zone of vacuoles beneath the 

 theca of Ceratium (53) also functions in flotation. 



CHROMATOPHORES, PIGMENTS, PYRENOIDS, 

 PHOTORECEPTORS 



Chromatophores 



Chromatophores, found in many phytoflagellates, vary in number, 

 size, color, and form in different groups. Some flagellates contain one 

 large cup-shaped chromatophore, or an H-type in which two large lobes 

 are joined by a connective (Fig. 1. 17, A, E, F, I). The chromatophores 

 in Peridininm umbunattim form an anastomosing network with lobes 

 extending into the endoplasm (Fig. 1. 17, J). Some of the Euglenidae 

 contain many small flattened chromatophores arranged in a peripheral 

 layer (Fig. 1. 17, G). 



Pigment-free leucoplasts, homologous with chromatophores, have been 

 reported in Polytoma (219) and Polytomella (185). Since Pringsheim 

 (183) has pointed out that peripheral mitochondrial networks have 

 sometimes been interpreted as leucoplasts, the status of leucoplast-bearing 

 species requires further investigation. 



Pyrenoids 



These structures (Fig. 1. 17, B, C, J, L), which are usually asso- 

 ciated with chromatophores, vary from solid bodies to aggregates of 

 granules, around which starch or paramylum may accumulate. Each 

 pyrenoid in Euglena americana (Fig. 1. 17, L), for example, consists of 

 two plano-convex masses adherent to the chromatophore and covered by 

 paramylum (88). The extraplastidial "pyrenoids" of certain Crypto- 

 monadida are termed amphosomes by Hollande (88), who believes that 

 homologous structures are unknown in other flagellates. The amphosome 

 of Cryptomonas dangeardii (Fig. 1. 17, N) consists of two chromophilic 

 plates separated by chromophobic material, and is sometimes surrounded 

 by starch grains. The functional significance of the amphosome is un- 

 known. 



Pyrenoids have been interpreted as reserves of protein, as structures 

 involved in the synthesis of polysaccharides, and even as intracellular 

 symbiotes. The second assumption is consistent with the frequent occur- 

 rence of starch or similar materials immediately surrounding the pyre- 

 noids. According to various reports, pyrenoids may be resorbed occasion- 

 ally, they may be reduced in number during formation of cysts and 

 zygotes, and they may arise by division or be formed de novo. Such appar- 

 ent variations in behavior are of little assistance in functional inter- 

 pretations. 



