80 PROTOZOOLOGY 



Chromatophore-bearing Protozoa usually possess also a stigma 

 (Fig. 31) or eye-spot. The stigma may occur in exceptional cases 

 in colorless forms, as in Khawkinea, Polytomella, etc. It is ordi- 

 narily situated in the anterior region and appears as a reddish or 

 brownish red dot or short rod, embedded in the cortical layer of the 

 cytoplasm. The color of the stigma is due to the presence of droplets 

 of haematochrome in a cytoplasmic network. The stigma is incapable 

 of division and a new one is formed de novo at the time of cell divi- 

 sion. In many species, the stigma possesses no accessory parts, but, 

 according to Mast, the pigment mass in Chlamydomonas, Pando- 

 rina, Eudorina, Euglena, Trachelomonas, etc., is in cup-form, the 

 concavity being deeper in the colonial than in solitary forms. There 

 is a colorless mass in the concavity, which appears to function as a 

 lens. In certain dinoflagellates, there is an ocellus (Fig. 107, c, d, g, h) 

 which is composed of amyloid lens and a dark pigment mass (melan- 

 osome) that is sometimes capable of amoeboid change of form. The 

 stigma is, in general, regarded as an organella for the perception of 

 light intensity. Mast (1926) considers that the stigma in the Volvo- 

 cidae is an organella which determines the direction of the move- 

 ment. 



References 

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Fortschr. Zool., Vol. 6. 

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protist., Vol. 3. 

 Brown, V. E. 1930 The Golgi apparatus of Amoeba proteus. Biol. 



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 1930 The Golgi apparatus of Pyrsonympha and Dine- 



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 Calkins, G. N. and F. M. Summers (editors). 1941 Protozoa in 



biological research. New York. 

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The wood-feeding roach Cryptocercus, its Protozoa, and the 



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