FLAGELLATE PROTOZOA 



POLYTOMA 



Water in which organic matter is decaying always contains 

 numerous small organisms of various kinds. Among these, when 

 decomposition is well advanced, there can be found with the aid 

 of the microscope minute, colourless organisms of a species known 

 as Polytoma uvella (F g. 13), which feed by absorbing from the 

 water through the surface of their bodies substances n solution 

 derived from the decaying matter. The body of a Polytoma 

 is an egg-shaped mass of protoplasm without any internal skeleton. 

 A pair of long protoplasmic lashes or flagella project from one 

 end ; by a backward lashing of 

 these it swims with a somewhat 

 jerky course, the end at which the 

 flagella are placed being forward. 

 The permanent shape of the body 

 is due to a thin cuticle ; that is, 

 not to a surface layer of the pro- ^^• 

 toplasm, but to a protective cover- 

 ing formed by secretion. It is 

 pierced by two pores for the 

 flagella. Two contractile vacuoles lie 



c. V. 



cu. 



s.g. 



Fig. 



13. — Polytoma iivella : three 



stages in ordinary fission. 



C.V., Contractile vacuole ; cu., cuticle 

 nucleus ; s.g., starch grains. 



nu. 



close behind the flagella and con- 

 tract alternately. There is one nucleus, placed somewhat behind 

 the middle, and there is sometimes a spot of red pigment situated 

 in the front part of the body. The hinder region contains 

 numerous starch granules. These must be formed by the proto- 

 plasm from substances absorbed in the food : they serve as a 

 reserve of nutriment, and are used up during starvation. Their 

 presence is interesting, for starch, though it is common in plants, 

 is rare in the protoplasm of animals, which, if they store carbo- 

 hydrates, usually do so in the form of glycogen. Together with the 

 spot of red pigment — which is an organ that enables small, 

 motile, green plants to find the sunlight which is necessary to their 

 mode Oi nutrition— the starch granules betray the fact that 

 Polytoma is at least closely related to plants. It is, to all intents 

 and purposes, a colourless Chlamydomonas, an organism which 



37 



