64 ELEMENTS OF BIOLOGY 



Reproduction in Amoeba. As in all other living organisms, 

 the growth of an amoeba continues at a gradually diminishing rate 

 until a certain maximum size is reached, which probably varies 

 with a number of conditions in the environment. Further increase 

 in the amount of living protoplasm is accompanied by a division 

 of the amoeba into two daughter amoebae. This is accomplished by 

 the elongation of the nucleus, which tends to become dumb-bell 

 shaped, accompanied by a similar change in shape of the entire 

 cell (Fig. i6). The nucleus divides, accompanied by a division of 

 the cell. In such a reproductive process the parent disappears. Hence 

 the amoeba in the absence of accident or unfavorable change in its 

 environment, is immortal, the processes of division restoring the 

 two daughter organisms to the characteristics of young systems. 

 This simple type of equal division, which is common among Pro- 

 tozoa, is called binary fission. 



Paramoecium. Other classes of Protozoa illustrate distinctly 

 higher types of organization of the various parts of the organism, 

 new and more efficient specializations of the protoplasm. The mem- 

 bers of the class Infusoria include protozoons which move by means 

 of cilia, short, fine fibrils extending outward from the surface, their 

 bases embedded in the ectosarc. These cilia must not be compared 

 nor confused with hair, for hair is found only among the highest 

 animals (mammals) and each hair consists of several layers of cells. 

 The cilia of Infusoria are specialized protoplasmic structures which 

 exhibit constant contractility. The contractions are synchronized and 

 their beat is in regular order, thus imparting a bullet-like motion 

 to the organism. In some the cilia occur over the entire surface of 

 the animal; these are termed holotrichia. In the hypotrichia, the 

 cilia are absent on the upper surface. In the heterotrichia the cilia 

 occur over the entire surface but are very much longer about the 

 mouth. In the peritrychia the cilia are absent except in the region 

 of the mouth. 



The fresh water holotrychous ciliate, Paramcecium caudatiim 



