42 MANUAL OF ANIMAL BIOLOGY 



Lying between the ectoplasm and endoplasm are two con- 

 tractile vacuoles, one near each end of the animal. Each con- 

 tractile vacuole consists of a large central spherical vacuole and a 

 number of radiating canals which open into it ; the whole forming 

 a star-shaped structure. 



B. Life Processes of Paramecium 

 1. Nutrition 



Paramecium exhibits a typical holozoic type of nutrition. The 

 food consists for the most part of various kinds of Bacteria to- 

 gether with a number of species of tiny Flagellates, all of which 

 are obtained from the surrounding fluid medium. The cilia lining 

 the peristome play an important role in capturing the food. The 

 coordinated beat of these cilia is so directed as to cause a continuous 

 current of water, containing the food materials, to sweep down 

 to the posterior end of the peristome. Here the particles pass 

 through the mouth opening into the tubular gullet. The particles 

 of food collect at the posterior end of the gullet and form a gastric 

 vacuole in the endoplasm which, when it has attained a certain 

 size as a result of the continued increase in the number of captured 

 food particles, is detached from the end of the gullet. This 

 apparently takes place as the result of a contraction of the sur- 

 rounding endoplasm. As soon as one gastric vacuole is detached 

 from the end of the gullet another one at once begins to form in the 

 same place. 



The fully formed and detached gastric vacuole, as a result of 

 the continual streaming movement, or cyclosis, of the endoplasm, 

 is moved in a definite path through the body. It is first carried 

 backward with the endoplasmic current and almost reaches the 

 posterior end of the animal. It is then carried forward along the 

 dorsal surface to the anterior end of the body, where the proto- 

 plasmic motion is again reversed, and the gastric vacuole passes 

 first ventrally and then posteriorly toward a definite anal spot 

 which is situated on the ventral surface of the body, near the 

 posterior end. Emphasis should be laid upon the fact that this 

 process of cyclosis, which results in a definite orderly movement 

 of the gastric vacuoles through the endoplasm of Paramecium, is 

 due to a streaming of the endoplasm, the food vacuoles playing 

 an entirely passive role. 



