UNICELLULAR ORGANISMS 63 



teria, the nutritious portions of which are absorbed by the body 

 and assimilated into new Paramecium substance. The un- 

 digested refuse portion is eventually discharged at the posterior 

 end of the body on one side. There is no perrrmnent opening 

 here, but whenever material is to be rejected a temporary 

 opening appears, at the point shown at Fig. 21 ex, and the refuse 

 material is discharged into the water. The process by which 

 food is used, including the absorption of oxygen and the excre- 

 tion of waste products, as well as the oxidation of the food 

 itself, is essentially identical with that in the Amoeba. As the 

 result of the process, the food material is eventually assimilated 

 into new Paramecium substance, and the animal grows, increas- 

 ing in size until it is ready for reproduction. 



Irritability. Paramecium is totally lacking in sensory organs 

 or in a nervous system, but like the Amceba it reacts to a variety 

 of stimuli. If an injurious stimulus is applied to one side of it, 

 the animal will reverse its cilia and move away from the irritat- 

 ing stimulus. It may move backward or it may turn its forward 

 end in any direction and move off to one side. It is attracted 

 by certain chemical stimuli and repelled by others. It is 

 affected by heat in the same way as the Amoeba. It is slightly 

 affected by an electric current, but is not affected by ordinary 

 light, although the so-called ultra-violet rays have an influence 

 upon it. These various reactions give to Paramecia an appear- 

 ance of conscious sensation, and it appears as if they had the 

 power of volition to enable them to avoid irritating or unpleasant 

 conditions. But the facts do not necessarily prove this, for it 

 is possible that these reactions are ^only mechanical responses 

 to stimuli, such as might be found in other machinery. The 

 responses, however, are so complicated, and so resemble those 

 of truly conscious animals, that it leads one to suspect that they 

 are actually conscious functions. 



Reproduction. The ordinary method of reproduction of the 

 Paramecium is by division (fission) similar to that of Amoeba, 

 although it is more complicated, since the animal is more com- 



