202 COMPARATIVE ANATOMY 



infusions of organic matter. In size it is somewhat smaller 

 than Copromonas and in shape almost perfectly oval, with a 

 pair of flagella of medium length projecting from one of its 

 ends. Both flagella are used equally in swimming, and the 

 animalcule darts with great rapidity through the water, pur- 

 suing a more or less definite course. The protoplasm of the 

 body is invested by a delicate but distinct membrane or en- 

 velope, which it usually fills and fits against so closely that 

 the latter is difficult to see. But if the organism is badly 



v / 



A 



Fi g- 43- 



Polytoma uvella. A, an individual showing the envelope and con- 

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

 contractile vacuoles, and posterior starch granules. B-F, succes- 

 sive binary divisions within the envelope giving rise to four young 

 Polytomae. 



nourished the protoplasm shrinks and the envelope becomes 

 obvious. This fact shows that the envelope is not a simple 

 differentiation of the most external layer of the protoplasm, 

 but a true protective covering secreted by, but distinct from, 

 the protoplasm. The two flagella project through pores in 

 the envelope. The chemical constitution of the envelope is 



