42 



PROTOZOA. 



The animal retracts its cilia, contracts its body to a globular mass, 

 and then secretes a transparent cyst which hardens and protects it 

 and enables it to survive in damp air. In water the contents of 

 the cyst divide into a number of parts, which attain freedom by the 

 bursting of the cyst, each one becoming a young animal. 



The rapidity of fission depends upon the temperature and upon 

 the food, and seems to be fairly constant for each species. Thus an 

 individual of Stylonychia pust.ulata, if well supplied with food, will 

 divide once in twenty-four hours in a temperature of 5 to 10 C., 



and once in twelve 

 hours if the tempera- 

 ture be from 10 to 

 15 C. Although the 

 rapidity of fission is 

 less in senile individ- 

 uals, there does not 

 appear to be any special 

 increase in it after con- 

 jugation. 



The Conjugation of 

 the Ciliata is generally 

 of a temporary nature. 

 Two individuals (rarely 

 more) apply themselves 

 together and acquire 

 protoplasmic continu- 

 ity. After a few hours 



FIG. 36. Vorticclla microstoma, after Stein, a, in process of , ill 



fission ; N nucleus, the mouth apparatus in each portion is tliey separate and lead 



formed afresh ; 6, fission is completed, one product of it their ordinary life. Ill 

 is set free after the formation of a posterior circlet of cilia ; 



wadoral zone of cilia; cc oesophagus ; c, Vorticella in process tne V Orticellines tlie 



of bud-like conjugation ; k the bud-like individuals (micro- conjugation ^ Complete 

 gametes) attached. 



and permanent. As a 



general rule conjugation takes place between two ordinary individuals 

 of the species, but in some cases the conjugating individuals, i.e., 

 the gametes, are specially produced by division from the ordinary 

 individuals. Thus Leucoplirys patvla divides rapidly several times, 

 and produces dwarfed forms uniformly ciliated and incapable of 

 taking nourishment. These small forms are the gametes. In the 

 Vorticellince the gametes differ ; one of the conjugating individuals 

 is the ordinary fixed form, and is called the megagamete, while the 

 other is a small free-swimming form, produced by two or three 



