CHAPTER VII 



THE SPOEOZOA, MONOCYSTIS MAGNA AND 

 MONOCYSTIS AGILIS 



THE Protozoa described in the previous chapters are free living 

 animals, moving about in search of various kinds of solid food 

 and ingesting it when found. Some few members of the 

 Rhizopoda, it is true, are parasitic and inhabit the alimentary 

 canal of various animals, including man, but they may be 

 considered as exceptional, as they do not exhibit any marked 

 structural modification or specialised mode of reproduction 

 in connection with the parasitic habit. The class Sporozoa, 

 on the other hand, consists exclusively of parasitic Protozoa 

 which infest the cavities or tissues of almost every class of 

 vertebrate and invertebrate animals. As is commonly the 

 case with parasitic animals, each species of sporozoon is, as a 

 rule, exclusively confined to some particular species of animal 

 called its host, and frequently to some particular organ or 

 tissue of that host. Some infest the internal cavities of the 

 body ; others are, at some period of their existence at least, 

 cell-parasites that is to say. they live inside tissue-cells 

 of their host. But whatever situation they may affect, all 

 Sporozoa are alike in this respect, that they do not ingest 

 solid food, but absorb the juices of their hosts by osmosis 

 through the whole surface of their bodies. They likewise 

 all resemble one another in being able to multiply rapidly by 

 means of spores. A spore is a minute reproductive body 

 produced, as a rule, by division of the parent form, and the 

 various kinds of spore-formation in this class afford instructive 

 examples of the adaptation of animals to different modes of 

 existence, for each kind is clearly suited to the dissemination 

 of the spores in such a manner as to ensure their transference 

 from host to host. 



The two species, Monocystis magna and Monocystis agilis, 

 form convenient examples for the study of Sporozoa, because 

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