PARAMECIUM 43 



During this movement of the gastric vacuoles, the process of 

 digestion of the food material takes place. The results of experi- 

 mental studies show that the chemical actions, which take place 

 in the digestion of food in the gastric vacuoles of Paramecium and 

 in many other Protozoa, are fundamentally the same as take 

 place in the digestion of food in higher forms of animal life. In 

 all cases the food is acted on by the secreted enzymes which render 

 it soluble and capable of being assimilated by the protoplasm. 



Paramecium does not show a definite selection of food as was 

 noted in Amoeba. Almost any minute particle, whether it be 

 food or not, which comes in the range of the current of water 

 caused by the beat of the cilia in the peristome, may be carried 

 along and deposited in a food vacuole. This can be well-illustrated 

 by adding a small amount of an indigestible substance, such as 

 powdered carmine or other finely ground material, to the fluid 

 which contains Paramecia. A microscopic study of such a prep- 

 aration shows that the indigestible carmine particles are collected 

 to form gastric vacuoles in just the same way as are food particles. 

 The carmine-filled gastric vacuoles are also detached from the end 

 of the gullet and carried in the normal way through the endoplasm, 

 finally ending at the anal spot, through which the indigestible 

 carmine particles are egested. 



2. Respiration and Excretion 



The basic features of respiration (income of oxygen and outgo of 

 carbon dioxide) and excretion (the elimination of metabolic wastes 

 from the cell) are common to every living cell of every type of 

 organism. Also in the unicellular animals, the mechanisms neces- 

 sary for performing these functions are much the same. Thus the 

 same methods are used in Paramecium as in Amoeba, which are 

 the excretion of liquid wastes through the contractile vacuole 

 apparatus and the interchange of gases at the surface of the body, 

 and probably also to some extent by means of the contractile 

 vacuoles. 



The two contractile vacuoles in Paramecium maintain practi- 

 cally constant positions, one near the anterior end and the other 

 near the posterior end of the body. The liquid wastes first collect 

 in the radiating canals which surround each contractile vacuole 

 and then pass from the former into the central cavity, from which 

 they are expelled to the exterior later by the contraction of the 



