284 SPOROZOA CHAP. 



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 differentiation 

 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 harmless, 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 Trypanosomd] 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 abundantly in earthworms. 

 When the body-wall of an earthworm is cut open, certain 

 white, lobed organs known as sperm-sacs are very apparent a 

 short distance from the anterior end of the animal (compare 

 Chapter VI.). In these, the sperms of the worm undergo 



