126 



INTESTINAL FLAGELLATES AND CILIATES 



Ciliates 



Balantidium coli. — Although several species of ciliates have 

 been recorded as human parasites, there is only one species, 

 Balantidium coli (Fig. 34A), normally parasitic in hogs, which is 

 common enough to be of any importance. This large ciliate 

 stands next to Endamoeba histolytica among the Protozoa as a 



Fig. 34. Balantidium coli; A, free ciliate from intestine; n., nucleus; c. v., 

 contractile vacuoles; f. v., food vacuole; cyt., cytostome. B, cyst, as passed in 

 faeces, containing two parasites. X about 500. (After Wenyon.) 



cause of human dysentery. It is a large animal for a protozoan, 

 averaging from 50 to 100 /^ {ziu to ^^^ of an inch) in length, and 

 thus being visible to the naked eye. Its body is oval and en- 

 tirely covered with ciUa, and at the anterior end there is a gash- 

 like slit leading to the mouth or " cytostome " (Fig. 34, cyt.). 

 The large bean-shaped nucleus (Fig. 34, n.) lies near the middle of 

 the body and near each end is a pulsating cavity or contractile 

 vacuole (Fig. 34, c.v.) which excretes waste matter. These 

 parasites multiply by transverse division, often so rapidly that 

 the animals do not have time to grow to full size and so become 

 very small. When ready to leave the body they form an oval 

 cyst about themselves. Sometimes two occupy a single cyst 

 (Fig. 34B), and later fuse together. Since the ciliated bodies of 

 the protozoans can be seen, under a microscope, inside the large 

 transparent cysts, their identification is not difficult. The cysts 

 can exist outside the body for a long time, awaiting an opportunity 

 for reinfection. 



