PARAMECIUM 



71 



A 



emptied. The two vacuoles empty alternately at intervals of about 10 

 to 20 seconds. 



Near the center of the animal or somewhat behind it, and not far 

 from the mouth, is the macro nucleus, which has somewhat \^ 



the form of a lima bean, and a very much smaller micronu- 

 cleus. The micronucleus is lodged in a depression on the 

 surface of the macronucleus at a point which would corre- 

 spond to the hilum of the bean, the place where the root 

 comes out in germination. 



100. Metabolism. — The food of paramecia consists of 

 bacteria and minute forms of Protozoa. It is swept into 

 the oral groove by the action of the cilia, carried back 

 through the mouth, onward into the gullet, and finally 

 passed into the endoplasm, where the food vacuole is formed. 

 The passage through the gullet is effected by means of two 

 or three undulating membranes, formed by rows of cilia 

 placed side by side and fused. These food vacuoles are in 

 constant circulation around the animal, following a definite 

 course. Digestion takes place in the food vacuoles as it 

 does in the ameba, and circulation, assimilation, and dis- 

 similation are similar. Excretions are accumulated in the 

 contractile vacuoles and eliminated through them to the 

 outside. Expiration also takes place into the contractile 

 vacuoles. Inspiration seems to be possible over the whole 

 body surface. Egestion occurs at a particular point near 

 the posterior end, where there is a potential opening 

 through the ectoplasm known as the anus, or anal spot. 



101. Locomotion. — Owing to the presence of the elastic 

 pellicle, the body of the Paramecium exhibits elasticity, 

 which is not observed in ameba. It can force its way 

 through a narrow passage, the body contracting as it does 

 so, but on its exit from the passage it immediately 

 assumes its normal shape. 



Locomotion is effected by means of the cilia, which 

 may beat either forward or backward and by means of 

 which the animal swims in either direction through the 

 water. Normally it moves forward. The cilia, however, 

 do not beat directly backward but obliquely, so that the 

 animal rotates on its long axis. The cilia in the oral groove 

 also strike obliquely along the axis of the groove and 



Fig. 20. — The spiral path followed by a swimming Paramecium. Fia. 20. 

 (Modified from Jennings, "Behavior of the Lower Organisms," by the 

 courtesy of Columbia University Press.) The rotation of the animal on its axis is indicated 

 by the position of the oral groove. The large arrow shows the direction of motion, and 

 the axis of the spiral. 



