THE PROTOZOA OR THE SIMPLEST ANIMALS 143 



meganucleus. Reproduction in Paramcecium takes place 

 by transverse fission. Both meganucleus and micro- 

 nucleus divide and the two parts pass toward either end 

 of the body which becomes constricted in the middle by a 

 transverse furrow and finally pinches in two. Para- 

 mcecium may reproduce in this way for several hundred 

 generations, but finally it is interrupted by another proc- 

 ess which is called conjugation. In this process the Para- 

 mcecia come together in pairs and become united by their 

 oral surfaces. When they are united complicated changes 

 in the nuclei occur, during which the Paramcecia exchange 



FIG. 117. Diagram of the course of a Paramoecium in a drop of dilute 

 acid surrounded by water. 



a part of their nuclear material. After this they separate 

 and continue dividing by fission until another conjugation 

 period comes about. It is held by some biologists that 

 conjugation regenerates, or puts new vitality into the 

 stock. 



The body of Paramoecium is asymmetrical, and as the 

 animal swims through the water by the Keating of the 

 cilia it rotates on its long axis and describes a spiral path. 

 When colliding with an object, or when stimulated in 

 any other way, Paramoecium swims backward by revers- 

 ing the stroke of the cilia, turns toward the side opposite 

 the mouth, and then goes ahead. This reaction has been 



