Phylum Protozoa 373 



Stentors. Didinium, another nonstalked form, feeds upon Paramecium 

 and is able in a short time to wipe out a culture. The stalked forms 

 include such genera as Vorticella and Carchesiuiu. Some of these 

 latter, such as Vorticella, have contractile fibris, primitive myonemes, 

 for the contraction and expansion of the stalk. 



Fig. 121. — Some ciliates and a suctorian. A, Didinium; B, Balantidium; C, Vorticella; 



D, Ephelota, a suctorian. 



Parasitic Forms. — This class also has a number of parasitic forms. 

 One of these, Balantidium coli, is found in the intestine of pigs. Oc- 

 casionally it occurs in man, probably because the cysts are accidentally 

 carried into the mouth. When this infection occurs, Balantidium is 

 able to invade the intestinal mucosa and causes severe symptoms. 



In the rumen of many of the ruminants (cattle, sheep, etc.) are 

 found ciliates of several genera, including Diplodinium. They ap- 

 parently act as commensals, living on the ingested cellulose of the 

 host. Structurally these are among the most complicated of the ciliates, 

 having special skeletal supporting structures, motor bodies, and re- 

 tractile fibrils. 



THE CLASS SUCTORIA 



The members of this relatively small class are considered by some 

 to be merely modified ciliates. The young possess cilia and resemble 

 closely the ciliates. As they mature, they lose their cilia and change 

 from the free-swimming state into attached forms. The cell body 

 is attached by a stalk or disc to the substratum. The body is variously 

 shaped in the different genera, and there is no cytostome. Tentacles 

 are present for the procurement of food. These are of two types ; one 



