Apicomplexans

Apicomplexans are those protozoa that are distinguished by an apical complex when viewed with an electron microscope. Furthermore, only the microgamete stages have flagellar organelles. They typically have complex life cycles, either direct (host-to-host) or indirect (host-to-intermediates-to-host). All of the apicomplexans that we'll examine belong to the Sporozoa (Sporozoasida), another common name for this group (but limited to those with direct life cycles).

Gregarinasina: Monocystis and allies
Easy to study, gregarines have been seen by most undergraduates as living examples of sporozoans. Monocystis sporocysts are ingested, releasing sporozoites which penetrate tissues and travel to their destination. They then transform into trophozoites, and, after feeding, become oocysts. These oocysts (containing gametocytes) undergo numerous divisions to form gametes. The gametes then fuse to form zygotes. The zygotes then encyst, forming sporocysts. The gametic cycle occurs entirely within the oocyst, and the sporocysts leave the host by decomposition of the host.

More recognizable gregarines are found among the Gregarina spp., which features a body division into a protomerite and deuteromerite. The deuteromerite will ultimately detach to form the sporadin, which unite with other sporadin under syzygy. These then encyst to form a gametocyte. Repeated nuclear division and subsequent cellular division produce gametes, which then fuse to from zygotes. The zygotes undergo division to form sporozoites, with the protomerite and deuteromerite.

Eucoccidiorida: Haemogregarina, Eimeria, Cryptosporidia, Toxoplasma, and allies
Coccidians are intracellular parasites, often producing coccidiosis in a number of wild and domestic animals. There are three phases in their life cycles. Merogony involves multiple asexual reproduction inside the host cells, forming merozoites. Gametogony occurs when the meronts of microgametes (flagellated) and macrogametes (aflagellar) fuse. Sporogony is the formation of the zygote and multiple fission to produce the sporozoites. The sporozoites infect the host cells.

Eimeria tenella, early schizont, 900x.

Eimeria tenella, second generation schizont, 900x.

Eimeria tenella, merozoites?, 900x.

Sarcocystis spp, 900x.

Cryptosporidia spp, 900x.

Toxoplasma spp, 900x.