174 



COMPARATIVE ANATOMY 



The two flagella project through pores in the envelope. The 

 chemical constitution of the envelope is not known, but it does 

 not give the reactions of cellulose. There is no mouth, but 

 two contractile vacuoles may be seen close to the bases of the 

 flagella. A nucleus is placed near the hind end of the body, 

 and generally a small stigma or nodule of hsmatochrome may 

 be distinguished in the anterior half of the body. Though 



Fig. 37- 



Polytoma ttvella. At an individual showing the envelope and con- 

 tained protoplasmic body, central nucleus, stigma, two flagella, two 

 contractile vacuoles, and posterior starch granules. B — j^, succes- 

 sive binary divisions within the envelope giving rise to four young 

 Polytomse. 



Polytoma has no chromatophors, is colourless and devoid of 

 chlorophyll, it has the power, usually possessed only by 

 organisms which contain chlorophyll, of storing up amylum in 

 its body. Close observation shows that the posterior half of 

 every well-nourished individual is filled with small granules, 

 which, when treated with iodine, give the characteristic reactions 

 of starch. If a Polytoma is starved by being transferred to a 

 liquid devoid of nutrient organic matter, the starch granules 



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