PHYLUM PROTOZOA. CILIATES 



59 



charge their contents to form the vacuole, 

 which in turn ejects the hquid to the ex- 

 terior. After each discharge a new contrac- 

 tile vacuole is formed. 



The rate of contraction varies with the 

 activity of the animal, temperature, and the 

 concentration of salts in the surrounding 

 water. Since the body of the paramecium is 

 surrounded by a semipermeable membrane, 

 and the concentration of water molecules 

 on the outside of the membrane is greater 

 than that inside as a consequence of dis- 

 solved substances in the protoplasm, water 

 is continually entering the cell. The con- 

 tractile vacuoles function primarily to re- 

 move the excess water and in so doing re- 

 move excretory waste products of metab- 

 olism from the body. 



Excretion 



Excretion is a type of activity in which 

 the waste products of metabolism are elimi- 

 nated from protoplasm. Most of the waste 

 products of paramecia appear to diffuse to 

 the exterior through the pellicle, but nitrog- 

 enous substances have been detected in the 

 contractile vacuoles, which may therefore be 

 excretory in function. 



Respiration 



This process in the paramecium corre- 

 sponds to the internal (cell) respiration of 

 man. Oxygen, dissolved in the water, diffuses 

 through the surface of the body into the 

 protoplasm, just as oxygen is taken from the 

 blood by our red blood corpuscles. Carbon 

 dioxide diffuses out of the body through the 

 surface, although the contractile vacuoles 

 probably discharge some of the carbon 

 dioxide as well as nitrogenous wastes. 



Locomotion 



The cilia are fine protoplasmic processes 

 that cover the body of the paramecium; the 

 effective strokes of all the cilia force the 

 animal forward or backward. Since they beat 

 obliquely, the animal rotates on its long 

 axis as it swims forward (Fig. 25). The 



swerving of the body away from the oral side 

 is due to the fact that the cilia in the oral 

 groove beat more strongly than others, but 

 the rotation of the animal on its long axis 

 enables it to follow a more or less straight 

 course in forming large spirals. 



The remarkable coordination of the cilia 

 is probably made possible by a mechanism of 

 tiny fibrils just beneath the pellicle which 

 connect one cilium with another (Fig. 26). 

 The fibrils concentrate at one point near the 

 cell gullet to form a "neuromotor" center. 

 If this center is experimentally destroyed, 

 the cilia fail to beat in a coordinated man- 

 ner; this results in the loss of coordination 

 in movements. 



Behavior 



As in the amoeba and the euglena, 

 changes in the environment serve as stimuli 

 to which paramecia respond in various 

 ways. 



Avoiding reaction 



One of the most common responses of 

 the Paramecium is known as the avoidmg 

 reaction (negative response) (Fig. 28). 

 When a free-swimming paramecium en- 

 counters a harmful chemical such as strong 

 salt, it may reverse its cilia and swim back- 

 ward for a short distance; then its rotation 

 decreases in rapidity, and it swerves toward 

 the aboral side more strongly than under 

 normal conditions. Its posterior end then 

 becomes a sort of pivot upon which the 

 animal swings in a circle. During this revolu- 

 tion, samples of the surrounding medium are 

 brought into the oral groove. Wlien a sam- 

 ple no longer contains the stimulus to which 

 it reacts negatively, the cilia resume their 

 normal beating and the animal moves for- 

 ward again. If this movement once more 

 brings it into the region of the harmful 

 chemical, the avoiding reaction is repeated; 

 this goes on as long as the animal receives 

 the stimulus to which there is a negative 

 reaction. 



