284 SPOROZOA CHAP. 



with cilia (I) : these conjugate with one another, and 

 the zygote becomes encysted. On emergence from the 

 cyst, as the young Opalina absorbs the digested food in 

 the intestine of its host, it grows, and at the same time 

 its nucleus divides repeatedly (K) in the way already 

 described, until by the time the animalcule has attained 

 the maximum size it has also acquired the large number 

 of nuclei characteristic of the genus. 



Here, again, we have an interesting case of differenti- 

 ation or development (p. 276 ): the organism begins life as 

 a very small uninucleate mass of protoplasm, and as it 

 increases in size, increases also in complexity by the 

 repeated binary fission of its nucleus. 



In another group of the Protozoa known as the 

 Sporozoa parasitism occurs without exception, and the 

 relation between parasites and host is much more 

 intimate than in the case of Opalina : instead of living 

 within the enteric canal and merely absorbing the 

 contained products of digestion, these penetrate into 

 the tissues of the host, even passing into the interior of 

 its cells, from the contents of which they absorb fluid 

 nutriment. As is the rule in parasites, they are able to 

 multiply very rapidly. 



Sporozoan parasites occur in most classes of animals : 

 many seem to be comparatively innocuous, so that even 

 when infested with large numbers the host apparently 

 suffers no harm. In other cases they, like some other 

 parasitic Protozoa (e.g., the flagellate Trypanosoma in 

 "sleeping sickness") and certain Bacteria, may cause 

 dangerous diseases and epidemics (compare p. 287). 



As an example which is easily obtainable we will 

 examine Monocystis (Fig. 74), one of those Sporozoa 

 commonly known as Gregarines, which occurs abun- 

 dantly in earthworms. When the body-wall of an 



