THE PROTOZOA 



191 



If it pushes its way past any dense substance 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 semifluid 

 protoplasm which has a granular grayish appearance under the 

 microscope. This protoplasm appears to be bounded by a very 

 delicate membrane through which project numerous delicate threads 

 of protoplasm called cilia. (These are 

 usually invisible under the microscope.) 



The locomotion of the paramoecium is 

 caused by the movement of these cilia which 

 lash the water like a multitude of tiny oars. 

 The cilia also send tiny particles of food into 

 a fmmel-like opening, the gullet on one side 

 of the cell. Once within the cell body, the 

 particles of food materials are gathered into 

 little balls within the almost transparent 

 protoplasm. These masses of food seem to 

 be inclosed within a little area containing 

 fluid, called a vacuole. Other vacuoles 

 appear to be clear; these are spaces in Paramoecium 

 which food has been digested. One or 

 two larger vacuoles may be found; these 

 are the contractile vacuoles; their purpose 

 seems to be to pass off waste material from 

 the cell body. This is done by pulsation 

 of the vacuole, which ultimately bursts, pass- 

 ing fluid waste to the outside. Solid wastes are passed out of 

 the cell in somewhat the same manner. The nucleus of the cell 

 is not easily visible in living specimens. In a cell that has been 

 stained it has been found to be a double structure, consisting of 

 one large and one small portion, called respectively the macro- 

 nucleus and the micronucleus. 



Response to Stimuli. — In the paramoecium, as in the one-celled 

 plants, the protoplasm composing the cell can do certain things. 

 Protoplasm responds, in both plants and animals, to ccTtain agen- 

 cies acting upon it, coming from without ; these agencies we call 

 stimuli. Such stimuli may be light, differences of temperature, 

 presence of food, electricity, or other factors of its surroundings. 



w.v: 



Greatly 



magnified. From aide. 

 F.V., food vacuole ; C.V., 

 contractile vacuole; 

 M, mouth ; A', nucleus ; 

 TT. T., water vacuole. 

 (After Sedgwick and 

 Wilson.) 



