THE PROTOZOA 135 



Parasitic Protozoa. 



The two protozoa just considered are free-living forms, that 

 is to say, they move about freely from place to place finding and 

 catching their own food. On the other hand, a large number of 

 the Protozoa do not do this, but live on, and in, other animals whose 

 tissues and juices they use as food. Animals living in this manner 

 are termed parasites, and the form upon which they live is called the 

 host. One whole class of the Protozoa, namely, the class SPOROZOA, 

 is composed entirely of parasitic species. The Sporozoa possess 

 certain characters in common. They all live inside the bodies of 

 their hosts and so are internal or endoparasites, some living in the 

 cavities of the viscera, but others actually inside the individual cells ; 

 the latter we distinguish as intracellular, and the former as inter- 

 cellular parasites. As a general rule, each species is limited to a 

 definite species of host to which in many, perhaps the majority of 

 cases, they appear to bring no harm, and so are described as harmless 

 parasites. In other cases they are very harmful, setting up a 

 diseased or pathological condition which produces great bodily 

 disturbances, and may even prove fatal. They all live on fluid 

 food which is absorbed by osmosis over the general surface of the 

 body, so that as a result they lack a mouth and a pharynx, either 

 permanent or temporary, also, as it is food already digested and even 

 assimilated by the host, they have no food vacuoles and no con- 

 tractile vacuoles. There is no need for them to move actively to 

 find nutriment, so that locomotor organs are absent. The name 

 of the class is derived from another of their principal characteristics, 

 and that is that reproduction takes place by the formation of spores. 

 In the process of spore formation, or sporulation, the parent indi- 

 vidual breaks up into a large number of minute nucleated fragments 

 of protoplasm, often protected in some way or other, which serve 

 for the dissemination of the species and the spreading of the parasites 

 to new hosts. Two members of the class we shall now study in some 

 detail, namely, Monocystis and Plasmodium. 



Parasitic Protozoa i. Monocystis. 



Two species of Monocystis, namely, M. magna and M. agilis, 

 are to be found in some stage of their life history in practically every 

 ordinary earthworm, and so serve conveniently for purposes of 

 study. They are both to be discovered in the sperm sacs of the 

 earthworm, large conspicuous whitish sacs lying in the ninth to 

 twelfth segments of its body, in which the sperms undergo part of 

 their development and are stored until required for use. M. magna 

 in the mature condition may reach a length of 5 mm., and so is 



