530 Host-Parasite Relationships 



The evolution of parasites 



Speculations concerning the origin and development of protozoan 

 parasites have been based upon certain assumptions and upon rather 

 limited observational and experimental data. Within the phylum, para- 

 sites are not limited to exclusively parasitic groups but are also scattered 

 in various orders which contain mostly free-living species. Such taxonomic 

 distribution suggests that parasitic Protozoa have arisen frequently and 

 independently from different groups of free-living ancestors. 



The origin of ectoparasites from free-living species may be assumed 

 as a matter of course. Endoparasites also may have arisen directly from 

 free-living ancestors. Another possibility is that endoparasites have 

 developed from ectoparasites whose prior origin was favored by the 

 minor adaptive changes required for the transition to ectoparasitism. 



As pointed out by Wenrich (98), it is difficult, with protozoological 

 data, to support the origin of endoparasites from ectoparasites. Ectopara- 

 sites include mostly primitive flagellates, certain Peritrichida and a num- 

 ber of Suctorea. Genera containing both ectoparasitic and endoparasitic 

 species are rare and it is difficult to trace possible connecting links. It is 

 more probable that endoparasites have arisen directly from free-living 

 species. The primary invasion probably led to colonization of the diges- 

 tive tract in most cases. Invasion of the blood and other tissues by many 

 species followed eventually in the course of evolution. Opportunities for 

 entering the digestive tract are certainly abundant enough, although 

 the primary invader must overcome new environmental hazards and must 

 also establish an infection if it is to succeed as a parasite. The latter step 

 involves satisfying food requirements and carrying on reproduction. 

 Furthermore, the probationary parasite must possess or develop methods 

 for insuring a safe passage from the first host to new ones if it is to 

 become anything more than a sporadic invader. 



It is often assumed that the original host of certain parasites, which 

 now have two hosts, was the one termed the intermediate host (or vector), 

 and that parasitism in the final host may be regarded as a secondary 

 adaptation. Whether this hypothesis can be applied to the genus Leish- 

 mania is somewhat uncertain. Among the species found in reptiles, 

 L. chamaeleonis is an intestinal flagellate retaining the leptomonad form 

 (100), while certain other species invade the blood of gekkos and are 

 found also in sandflies (Chapter XII). 



The occasional occurrence of sporadic endoparasitism by normally 

 free-living species suggests that direct transition may not have been too 

 difficult for some Protozoa. Sporadic paratism by Euglenida has been 

 reported in tadpoles (36, 97) and millipedes (98). Experimental infection 

 with Tetrahymena pyriformis has been established in the haemocoel of 

 insects (47, 65). Natural invasion by this ciliate or related species has been 



