120 



PHYLUM PROTOZOA — THE SIMPLEST ANIMALS 



debris is passed out. The cilia are arranged so as to waft 

 food particles into the mouth and down the passage. 

 There is a large and horseshoe-shaped macronucleus, 

 and a small micronucleus. Food vacuoles and contractile 

 vacuoles are present as usual. 



Sometimes a Vorticella bell jerks itself off its stalk and 

 swims about ; in other conditions it may form a temporary 



-nig- 



4 

 5- 



Fig. 53. — Vorticella. — After Biitschli. 



Structure. A'^., Macronucleus ; «., micronucleus ; c.v., con- 

 tractile vacuole ; m., mouth ; f.v., food vacuole ; v., vestibule. 

 Encysted individual. 3. Division. 



Separation of a free-swimming unit — -the result of a division. 

 Formation of eight minute units (tng.). 



6. Conjugation of microzooid (mg.) with one of normal size. 



cyst ", normally, the cilia are very active, and the move- 

 ments of the stalk frequent and rapid. Multiplication may 

 take place by longitudinal fission — a bell divides into 

 similar halves ; one of these acquires a basal circlet of cilia 

 and goes free, ultimately becoming fixed. Or the division 

 may be unequal, and a microzooid, or as many as eight, 

 may be set free. These swim away by means of the 

 posterior girdle of cilia, and each may conjugate with an 



