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Apicomplexa is a phylum in the kingdom Protista; they are spore-forming single-celled parasites of animals.• This large group of protozoa includes such diverse organisms as coccidia, gregarines, piroplasms, haemogregarines, and the malarias. Although they may not have identical life histories, they are all characterized by the presence of unique subcellular organelles known as an "apical complex": polar rings, rhoptries, micronemes, and often a conoid (a hollow cone of spirally coiled filaments in the organism's anterior tip) -- visualized only by electron microscopy.• They are exclusively parasitic and generally lack flagella or pseudopods, except for certain gamete stages. Diseases caused by apicomplexan organisms include: Most members have a complex life-cycle, involving both asexual and sexual reproduction. Typically, a host is infected by ingesting cysts, which divide to produce sporozoites that enter its cells. Eventually, the cells burst, releasing merozoites which infect new cells. This may occur several times, until gamonts are produced, forming gametes that fuse to create new cysts. There are many variations on this basic pattern, however, and many Apicomplexa have more than one host. The apical complex includes vesicles called rhoptries and micronemes, which open at the anterior of the cell. These secrete enzymes that allow the parasite to enter other cells. The tip is surrounded by a band of microtubules, called the polar ring, and among the Conoidasida there is also a funnel of rods called the conoid. Over the rest of the cell, except for a diminished mouth called the micropore, the membrane is supported by vesicles called alveoli, forming a semi-rigid pellicle. The presence of alveoli and other traits place the Apicomplexa among a group called the alveolates. Several related flagellates, such as Perkinsus and Colpodella have structures similar to the polar ring and were formerly included here, but most appear to be closer relatives of the dinoflagellates. They are probably similar to the common ancestor of the two groups. Another similarity is that apicomplexan cells contain a single plastid, called the apicoplast, surrounded by either 3 or four membranes. Its functions are thought to include tasks such as lipid synthesis, it appears to be necessary for survival. They are generally considered to share a common origin with the chloroplasts of dinoflagellates, although some studies suggest they are ultimately derived from green rather than red algae. The Apicomplexa comprise the bulk of what used to be called the Sporozoa, a group for parasitic protozoans without flagella, pseudopods, or cilia. Most of the Apicomplexa are motile however. The other main lines were the Ascetosporea, the Myxozoa (now known to be derived from animals), and the Microsporidia (now known to be derived from fungi). Sometimes the name Sporozoa is taken as a synonym for the Apicomplexa, or occasionally as a subset.
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