Host-Parasite Relationships 533 



animals; malarial parasites of birds generally can parasitize a variety of 

 avian hosts; Balantidium coli occurs naturally in man, apes, monkeys, and 

 pigs; Toxoplasma, recovered from man, is infective for a number of 

 mammals (70). 



TAXONOMIC DISTRIBUTION OF 

 PARASITIC PROTOZOA 



The Phytomastigophorea are represented by only a few parasitic 

 species and the authentic cases apparently are limited to two orders. The 

 Dinoflagellida include about 15 genera of parasites. Parasitic Euglenida 

 are represented by Euglenamorpha (97) and Hegneria (12) and by several 

 species of Euglena and Astasia. In contrast to the Phytomastogophorea, 

 many Zoomastigophorea are parasitic — the orders Hypermastigida and 

 Trichomonadida and a number of smaller groups are exclusively para- 

 sitic. 



Among the Sarcodina, the Proteomyxida include a few parasitic species 

 and a number of the Mycetozoida also are parasitic. The Endamoebidae 

 are all parasitic, and Wenyon (100) has suggested that every vertebrate 

 species probably will be found to harbor parasitic amoebae. The majority 

 of these amoebae seem to be endocommensals. However, man is not the 

 only host of a pathogenic species, since reptiles (81a) also may suffer 

 from amoebiasis. 



All known Sporozoa are parasitic and the majority cause appreciable 

 damage to their hosts. Certain groups have become adapted to particular 

 environments within the host. The Gregarinidia live primarily in such 

 cavities as the digestive tract and coelom of invertebrates. The Coccidia 

 are mainly invaders of epithelial cells, while the Haemosporidia occur 

 in blood cells and, as exoerythrocytic stages, in certain other tissue cells 

 of vertebrates. 



Among the Ciliatea, the Protociliatia, Astomina, and Entodiniomor- 

 phina are exclusively parasitic. In addition, a number of parasitic genera 

 and species are scattered among the rest of the ciliates. Some parasitic 

 ciliates, such as Ichthyophthirius multifiliis and Balantidium coli, nor- 

 mally invade and destroy tissues of the host. Many others appear to be 

 commensals, while certain ciliates of ruminants have been considered 

 possible symbiotes. The Suctorea include only a few ectoparasites, pre- 

 sumably ectocommensals, and a few endoparasites. 



PROTOZOA AS HOSTS 



In addition to their representation among parasites, Protozoa 



also serve as hosts of microorganisms.^ Hyperparasitism, in which parasitic 



Protozoa are invaded by their own parasites, is not uncommon (57, 86). 



Some combinations, involving algae in free-living Protozoa, are possibly 



" An extensive review of this subject has been published by Kirby (57). 



