CHAPTER XX 



ORGANISMS LIVING ON AND IN PROTOZOA 



Harold Kirby, Jr.' 



Protozoa as a group may be hosts of a great variety of other organisms. 

 Some of these are epibiotic, and in them the relationship ranges from 

 occasional phoresy to obligatory and constant association. True ectopara- 

 sitism exists in some epibionts, though often the distinction from preda- 

 tism is disputable. Protozoa are not so constituted as to be capable of 

 harboring inquilines; all endobionts are intracellular. Unless autotrophic, 

 therefore, endobiotic forms are parasites in the sense that they are de- 

 pendent on their hosts in nutrition. In many instances, however, the 

 protozoan suffers no apparent detriment from the relationship; and 

 sometimes the association of host and symbiont is constant. (Symbiosis 

 is used as a collective term, including commensalism, mutualism, and 

 parasitism.) It may be, even, that there is mutual advantage; but only 

 autotrophic forms are in a position to confer the commonest type of 

 benefit in mutualism, a nutritive one. It has been suggested, although 

 not demonstrated, that certain intracellular micro5rganisms may produce 

 enzymes that function in the nutritional metabolism of the host. Many 

 endobionts are more or less destructive parasites, which cause injury 

 or death; that is true, so far as is known, of all that invade the nucleus. 

 When the association has an obligatory and constant character, as in 

 the occurrence of bacteria on the body surface of certain Protozoa, or 

 of bacteria present in certain areas of the cytoplasm of all or almost 

 all specimens, the error has often been made of interpreting the sym- 

 bionts as structures of the host. When they are present only occasionally, 

 they have sometimes been mistakenly regarded as representing occasional 

 phases in the life history of the host, that is, reproductive phases. In- 

 creasing knowledge of the symbionts of the Protozoa has corrected most 



' Assistance rendered by personnel of Work Projects Administration, Official Project 

 number 65-1-08-113, Unit Cl, is acknowledged. 



