INFUSORIA 199 



that four micronuclei are present in each one of the two individuals 

 forming the couple. Three of these nuclei perish and become 

 absorbed, the fourth gradually passes to the portion of protoplasm 

 connecting the two conjugants, which has originated by absorption 

 of the cuticle at the point of contact of the conjugants. After a 

 further division one micronucleus of each conjugant passes over 

 into the other conjugant, and fusion ensues between the two micro- 

 nuclei of each individual. Complicated changes and divisions may 

 occur, but only the main principles can be noted here. A new nuclear 

 body is thus formed in each conjugant, and soon divides into two. 

 Of the segments thus produced one becomes a micronucleus, and 

 one or several of the others, as the case may be, form or amalgamate 

 into a new macronucleus, the old macronucleus usually perishing or 

 becoming absorbed during the conjugation. Usually, sooner or later, 

 the two conjugants separate, or may have separated already, and again 

 multiply independently by fission until a series of divisions by simple 

 fission is again followed by conjugation. The theoretical signifi 

 cance of conjugation cannot be dealt with fully here. It may be 

 remarked, however, that the macronucleus plays no part in it, but 

 governs entirely the metabolism of the Infusorian, whereas the 

 micronucleus is essentially a generative nucleus from which macro- 

 and micro-nuclei are again and again produced. 



Encystment amongst the Infusoria is very general, and is essentially 

 a means of protection when the surrounding medium dries up. 

 Doubtless these cysts are frequently carried long distances by the wind, 

 which explains the wide geographical distribution of most species. 

 Also, multiplication often takes place in the encysted condition. 



Some Infusoria live a free life, others are sedentary; the latter 

 form colonies in fresh as well as in salt water. Numerous species 

 are parasites of various 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, artificial, it is a convenient system. Biitschli has compiled a 

 better one. But for our purpose Stein's system is sufficient : 



Order i. Holotricha, Infusoria with cilia that are evenly distributed over the 

 entire body. 



1 It may be stated that numerous peculiarly shaped species live in the stomach of rumi- 

 nants, 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 exist 

 in and on the bodies of invertebrate animals. 



2 Bronn's Cl. u. Ordn. d. Thierr., i, Protozoa, Part 3, Infusoria. 



