532 Host-Parasite Relationships 



sporogony of Coccidia and Haemosporidia. The dinoflagellate, Amy- 

 loodinium ocellatiim (74), and the ciliate, Ichthyophthirius midtifiliis 

 (69), also undergo a period of rapid fission following prolonged growth. 

 In general, such morphological pecularities should perhaps be considered 

 adaptive features which have been preserved and augmented through 

 natural selection. 



One of the most interesting phases of adaptation to parasitism, that of 

 physiological and biochemical modifications, must await exploration 

 until more is known about food requirements and metabolism of para- 

 sites. This field of investigation may be expected to yield clues to funda- 

 mental factors in the evolution of parasites and in the maintenance of 

 more or less obligatory parasitism. 



Host-specificity 



The development of host-relationships has shown two general 

 trends (99). In various instances, small groups of parasites have become 

 adapted to a wide variety of hosts. Examples are found in different 

 groups of Protozoa. Species of Trypanosoma parasitize some five hundred 

 different species of vertebrates (100) and the genus Entamoeba also is 

 represented in many hosts. The genus Eimeria includes more than two 

 hundred species distributed among such hosts as annelids, insects, myria- 

 pods, fishes. Amphibia, reptiles, birds, and mammals (62). 



In the second type of development, a small group of parasites has 

 become restricted to a few hosts and may, in some cases, have undergone 

 extensive evolution within this limited environment. The Entodinio- 

 morphina contain many ciliates living in ruminants and in the cecum of 

 horses. Extensive speciation has been noted in some hosts. For Bos indicus, 

 13 genera containing about 100 species have been listed (58), while nine 

 species of Cycloposthium have been described from the horse (42). Cer- 

 tain genera of Hypermastigida and Trichomonadida also have undergone 

 extensive speciation in a limited group of termites (53), and the opalinid 

 ciliates are limited almost entirely to Amphibia (72). 



The host-specificity of individual species ranges from well-marked to 

 relatively slight in different cases. The Coccidia of mammals, in experi- 

 mental cross-infections, generally show a high degree of specificity (8), 

 although Isospora felis and /. rivolta can infect both cats and dogs (2). 

 The malarial parasites of man also show a fairly rigid host-specificity, 

 except for reports that they occasionally produce mild infections in experi- 

 mentally inoculated apes. Perhaps to a lesser degree, species of Giardia 

 may be restricted in their distribution among mammalian hosts (37). At 

 the other extreme, a species may be adaptable to a wide variety of hosts — 

 Herpetomonas inuscariun may invade flies belonging to a number of 

 different genera (6, 24); Trypanosoma brucei occurs in various wild and 

 domesticated mammals and may be transferred to certain laboratory 



