The members of the class Toxoplas- 

 masida have been and still are a headache 

 to taxonomists. Their affinities to other 

 protozoa are uncertain, and some people 

 have even questioned whether some of 

 them are protozoa at all, preferring to 

 assign them to the fungi. Until recently, 

 no relationship was recognized between 

 Sarcocystis and Toxoplasma, the 2 main 

 genera, but it has become increasingly 

 clear that they have many resemblances 

 (Manwell and Drobeck, 1953). Even the 

 names given to the trophozoites and to the 

 order have been wrong; they were influ- 

 enced by the belief (shown to be mistaken 

 by Perrier as long ago as 1907) that the 

 trophozoites are spores. Much of our 

 difficulty is due to lack of information. 

 As we learn more and more, and as new 

 facts fall into place, our understanding of 

 the group will improve and we can expect 

 that some of our present ideas may change. 

 The classification adopted here is consid- 

 ered reasonable and useful, but it is not 

 necessarily definitive. 



All members of this class are para- 

 sitic. They have no spores. They produce 

 cysts or pseudocysts containing many naked 

 trophozoites (sometimes called schizo- 

 zoites, altho the existence of schizogony is 

 dubious, and often erroneously called 

 spores or sporozoites). They are mono- 

 xenous and reproduce asexually. They 

 lack pseudopods, flagella and cilia, and 

 move by body flexion or gliding. 



There is a single order, Toxoplas- 

 morida, with the characters of the class. 

 It contains 2 families, Sarcocystidae and 

 Toxoplasmatidae, both of which contain 

 parasites of domestic animals and man. 

 (It is possible that the distinction between 

 these families is artificial, but, pending 

 further research, it is probably best to 

 retain them. ) 



C/fapter 12 



SARCOCYSTIS 



TOXOPLASMA 



Am RELATED 



PROTOZOA 



FAMILY SARCOCYSTIDAE 



Members of this family form cysts. 

 They multiply by binary fission and 



317 



