98 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



with a life cycle also involving an arthropod host. It is recognized 

 by Grasse as a sporozoan but not allocated to any recognized class. 

 It may be noted here that in an unillustrated abstract Rudzinska 

 and Trager (1960) describe it as being quite similar to Plasmodium 

 in ultrastructure. Like the latter it apparently ingests host 

 cytoplasm, but it does not produce hematin-like pigments as a 

 digestive product. 



The Class Sarcosporidea contains the single genus Sarcocystis, of 

 uncertain affinities and incompletely known life cycle. It is a 

 common parasite in the striated muscles of mammals, birds, and 

 reptiles, where large cysts are found, containing tremendous 

 numbers of banana-shaped organisms. Ludvik has used electron 

 microscopy to study Sarcocystis tenella, parasitic in the sheep (1956, 

 1958), and S. miescheriana from the pig (1960). Except for the 

 structure of the cyst wall, the two species are very similar, and 

 Ludvik's interpretation is summarized in Text-fig. 5. The body 

 is covered by a double membrane, which is thickened at the 

 anterior apex to form a polar ring. Within or beneath the pellicle 

 22 to 26 fine fibrils radiate from the polar ring and pass to the 

 posterior end of the cell. Immediately beneath the polar ring is 

 an internal organelle called the conoid, a truncate, funnel-shaped, 

 dense structure. The anterior third of the body is filled with 

 parallel longitudinal fibers, moderately dense and about 50 m/x in 

 diameter, arranged in orderly rows. Behind this zone is an accumu- 

 lation of large, dense, spherical granules of unknown composition. 

 The posterior third of the cell contains a granular cytoplasm 

 enclosing the nucleus, polysaccharide granules in vesicles, clear 

 vacuoles, and several large mitochondria. Since growth as well 

 as reproduction occurs within the cyst, exchange of material with 

 the host muscle tissue must be possible. In the sheep parasite the 

 outer layer of the cyst wall is spongy, the inner one broader and 

 uniformly finely granular. The wall of the pig cyst consists of a 

 rather thin layer of fibrogranular material resembling cytoplasm, 

 and long villi containing fine, wavy, longitudinal tubular fibrils 

 in a low-density matrix. These villi penetrate the host muscle 

 tissue to a depth of some microns. 



Among the parasitic organisms supposed to be protozoa but 

 unclassifiable on present evidence, none has received more 

 attention than Toxoplasma (see Ball, 1960). Unlike any other 



