INTROD UCTION 



Commensalism is an association in which one partner, the 

 commensal, is benefited, while the other, the host, is neither 

 injured nor benefited. A number of Protozoa live attached 

 to other Protozoa or other animals. For example, numerous 

 species of Suctoria are attached to the integument of active 

 aquatic animals. 



Parasitism is a somewhat different type of association. Here 

 a protozoan lives at the expense of another organism. The 

 former is the parasite and the latter the host. The injury to 

 the host varies greatly according to circumstances. Parasitic 

 Protozoa are found to occur in almost every phylum of the 

 animal kingdom, including phylum Protozoa itself. A few 

 forms are parasitic in plants. Some of the Suctoria are parasitic 

 in various Ciliophora. Microsporidia parasitize Myxosporidia 

 and Gregarinida, although they are ordinarily parasites of the 

 Metazoa. 



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Fig. 2 Encystment of Lophomonas hlattarum as seen in stained preparations. 



XI 150 (After Kudo). 



A precise distinction between the commensal and the para- 

 site is impossible, since in many cases there is no way to 

 determine the exact effect of the presence of the organism con- 

 cerned upon the host body. In its broad sense, the term para- 

 sitic Protozoa includes the commensals also. The Protozoa 

 inhabiting the digestive tract of the host encyst under certain 

 circumstances (Fig. 2). The cysts are voided in the fecal matter 

 and become the source of new infection. In other gut-inhabiting 



