35 kilobase circular genome (with the exception of Cryptosporidium species and Gregarina niphandrodes which lack an apicoplast). All members of this phylum have an infectious stage—the sporozoite—which possesses three distinct structures in an apical complex. The apical complex consists of a set of spirally arranged microtubules (the conoid), a secretory body (the rhoptry) and one or more polar rings. Additional slender electron-dense secretory bodies (micronemes) surrounded by one or two polar rings may also be present. This structure gives the phylum its name. A further group of spherical organelles is distributed throughout the cell rather than being localized at the apical complex and are known as the dense granules. These typically have a mean diameter around 0.7 μm. Secretion of the dense-granule content takes place after parasite invasion and localization within the parasitophorous vacuole and persists for several minutes. Flagella are found only in the motile gamete. These are posteriorly directed and vary in number (usually one to three). Basal bodies are present. Although hemosporidians and piroplasmids have normal triplets of microtubules in their basal bodies, coccidians and gregarines have nine singlets. The mitochondria have tubular cristae. Centrioles, chloroplasts, ejectile organelles, and inclusions are absent. The cell is surrounded by a pellicle of three membrane layers (the alveolar structure) penetrated by micropores. Replication: Mitosis is usually closed, with an intranuclear spindle; in some species, it is open at the poles. Cell division is usually by schizogony. Meiosis occurs in the zygote. Mobility: Apicomplexans have a unique gliding capability which enables them to cross through tissues and enter and leave their host cells. This gliding ability is made possible by the use of adhesions and small static myosin motors. Other features common to this phylum are a lack of cilia, sexual reproduction, use of micropores for feeding, and the production of oocysts containing sporozoites as the infective form. Transposons appear to be rare in this phylum, but have been identified in the genera Ascogregarina and Eimeria. Life cycle Most members have a complex lifecycle, involving both asexual and sexual reproduction. Typically, a host is infected via an active invasion by the parasites (similar to entosis), 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. Many variations occur on this basic pattern, however, and many