120 THE ANIMAL PARASITES OF MAN 



independently by segmentations until a series of segmentations are 

 again followed by conjugation. I cannot here go into the theoretical 

 signification of conjugation ; only this may be remarked, that the 

 macronucleus plays no part in it, but governs entirely the meta- 

 bolism of an infusorian, whereas the micronucleus is essentially a 

 sexual nucleus from which macro- and micro- nuclei are produced 

 again and again. 



Encystment amongst the infusoria is very general, and is 

 essentially a means of protection when the waters dry up ; no 

 doubt these cysts are frequently carried long distances by the 

 wind, which explains the wide geographical distribution of most 

 species. Frequently, also, reproduction takes place in the encysted 

 condition. 



Some infusoria live a free life, others are stationary, the latter 

 form colonies in fresh as well as in salt water ; numerous species 

 are parasites of various kinds of the lower and higher animals, 1 

 and a few also are parasitic in man. 



The Prague zoologist, v. Stein, introduced a classification of the 

 infusoria that has been almost universally adopted. It is founded 

 on the different position of the cilia on the body ; though, no 

 doubt, very artificial, it is a convenient system. Biitschli has com- 

 piled a better one (Bronn's Cl. u. Ordn. d. Thierr., vol. L, Pro- 

 tozoa, Part 3, Infusoria). But for our purpose Stein's system is 

 sufficient : 



Order i. Holotricha, infusoria with cilia that a're evenly distributed over 



the entire body. 

 Order 2. Heterotricha ciliated all over like the Holotricha, but have 



stouter cilia about the peristome. 



Order 3. Hypotricha, ciliated only on the ventral surface. 

 Order 4. Peritricha, with only a ring of spiral cilia, mostly stationary. 



The infusoria observed in man belong to the order of the 



Heterotricha . 



Genus i. Balantidium, Clap, et Lachm. 



Heterotrichous infusoria of oval or sac-like form and almost circular on 

 transverse section ; the anterior extremity narrowed, the posterior end 

 broad and rounded off, or also narrowed ; the peristome starting at the 



1 I may briefly state that numerous peculiarly-shaped species live in the stomach 

 of ruminants, others in the colon of horses ; several species are found in the rectum 

 of frogs and toads ; others, again, on the surface of the bodies of fishes ; and various 

 other species find the conditions for their existence in and on the bodies of invertebrate 

 animals. 



