THE PARAMECIUM 219 



lect in the contractile vacuole, which will burst and expel them. 

 The cell absorbs oxygen from the air in the water by diffusion, 

 and passes out carbon dioxide. Thus respiration takes place 

 through every part of the outer covering of the cell. 



The amoeba reproduces by splitting into two cells, each of which 

 resembles the parent cell, except that it is of smaller size. When 

 these new cells become the size of the parent amoeba, they each 

 divide. This is a kind of asexual reproduction. When conditions 

 unfavorable for life come, the amoeba, like some one-celled plants, 

 encysts itself within a membranous wall, forming a sporelike 

 structure. Upon return of favorable conditions, the cover dis- 

 appears and life begins again, as before. 



Laboratory Exercise. To learn something of the activities of a one- 

 celled animal, Paramecium. Use material taken from the surface of 

 a hay infusion made by placing a little hay in a beaker of water and 

 letting the material stand for several weeks. Place a drop of the infu- 

 sion on a slide, cover with a cover glass, and mount it under a com- 

 pound microscope. 



Do the moving structures appear to have any definite shapes ? Do 

 they move with any definite end forward? Do they collect in any 

 locality? If so, what influences them to do this? 



Heat a needle and introduce it at one side of the cover glass. Is 

 there any movement on the part of a Paramecium? 



Notice some of the animals grouped around small masses. Why do 

 you suppose they are there? Notice other animals with reference to 

 the position of air bubbles or to threads of Spirogyra to which they are 

 attached. How do they lie with reference to the air bubble? What 

 might the animal get from the air bubble, if it is to do work? How 

 would a cell covered with a membrane take anything from an air 

 bubble? What might it give in exchange? 



Drop a little fountain-pen ink on a slide containing Paramecia. 

 Note the long structures (trichocysts) thrown out by the animal. 



Write a paragraph explaining how a Paramecium reacts to the stim- 

 uli in its environment. Make drawings to illustrate your conclusion. 



The Paramecium. This one-celled animal is elliptical in outline, 

 but somewhat flattened. The rounded (anterior) end of the 

 body usually goes first. As the Paramecium pushes its way 

 between dense substances in the water, the cell body is seen to 

 change its shape as it squeezes through. 



The cell body is almost transparent, and consists of semi- 

 fluid protoplasm bounded by a very delicate membrane, the pellicle, 



