THE PROTOZOA 139 



There are two contractile vacuoles, one close to each end of the 

 body, while six to ten radiating canals communicate with these vacuoles 

 and other portions of the body. These canals collect the fluid from the 

 surrounding protoplasm and pour it into the vacuoles, after which the 

 vacuoles contract alternatively at intervals of twenty seconds and dis- 

 charge their contents to the outside of the body. 



This, it will be remembered, is as it is in Amoeba, where the con- 

 tractile vacuoles act as organs of both excretion and respiration. Para- 

 mcecia feed on bacteria and minute unicellular ani- 

 mals. The animal moves back and forth rapidly, caus- 

 ing a current of water to be sent down the gullet so 

 that various food particles are swept in. Along the 

 gullet there is a row of cilia which have fused together 

 forming what is called an undulating membrane. As 

 the food enters the end of the gullet a food vacuole is 

 moedum de produced, which as soon as fully formed, separates 

 fending itself from an from the gullet and is swept awav bv the rotary 



attack by a Proto- . " . J 



zoon, Didinium. The streaming movement of the endoplasm. i his process 



triochysts are dis- . . , N ,-,,,. ,. 



charged and mechan- IS known as CyClOSlS ( ). ihe digestion 



my lly away e ^From occurs within the food vacuole, while the undigested 

 Hegner, after Mast.) p ar ti c les are cast out at a definite anal spot which 

 can only be seen when these particles are discharged. 



BEHAVIOR 



Conjugation and division of Paramoecia will be discussed in the 

 following chapter. Here the ordinary reactions of this animal will be 

 taken up. 



While Paramoecia normally swim by means of cilia, they can, when 

 forced to, exhibit great elasticity and pass through very small openings. 

 The body goes forward, turning round and round on its long axis, al- 

 ways toward the left as it is propelled forward. This is the result of 

 the cilia in the oral groove growing more rapidly and effectively than 

 elsewhere, so that one obtains approximately the same effect as rowing 

 in a boat in which the oars on one side are applied much more strongly 

 than on the other. The animal would naturally swim in a circle if this 

 were the only force applied, but as it rotates on its long axis continu- 

 ally, it goes forward. This produces a spiral course. "The swerving, 

 when the oral side is to the left, is to the right ; when the oral side is 

 above, the body swerves downward; when the oral side is to the right, 

 the body swerves to the left, etc. Hence the swerving in any given 

 direction is compensated by an equal swerving in the opposite direc- 

 tion ; the resultant is a spiral path having a straight axis." 



Paramoecium responds to stimuli negatively and positively just as 

 do other forms of unicellular animals. This animal has been particu- 

 larly well studied in the laboratory as to its reactions to various stimuli, 



