CHAPTER VII 



POLYTOMA 



Water in which organic matter is decaying always contains 

 numerous small organisms of various kinds. 

 Features. Among these, when decomposition is well ad- 

 vanced, there can be found with the aid of the 

 microscope minute, colourless animals of a species known 

 as Polytoma uvella, which feed by absorbing from the water 

 through the surface of their bodies substances in 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 definite but 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, 

 which is not a surface layer of the protoplasm, but a 

 protective covering formed by secretion. It is pierced 

 by two pores for the flagella. Two contractile vacuoles 

 lie close behind the flagella and contract 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 

 protoplasm from substances absorbed in the food : they 

 serve as a reserve of nutriment, and are used up during 

 starvation. Their presence is remarkable, for starch, though 

 it is common in plants, is rare in the protoplasm of 

 animals, which, if they store carbohydrates, usually do so 

 in the form of glycogen. The animals can encyst, and 

 in the encysted state are carried about in dust, etc., to 

 germinate in favourable circumstances elsewhere. 



