The phylum Apicomplexa is composed entirely of parasitic single celled organisms. They are parasites primarily of animals, but often have an invertebrate vector. Most of the species have an apicoplast and apical organelle, which are used to enter the host cell. The life cycle involves three distinct stages, merogony(schizogony), gamogony, and sporogony. Transmission is via spores (sporozoa).
The apicomplexan life cycle varies between species. Generally, it is fairly complicated, and includes three distinct cycles, including merogony, gamogony, and sporogony.
Merogony: Following transmission, the parasite sporozoite infects a host cell and undergoes a period of asexual replication, or mitosis, forming a characterisitic number of small merozoites within a schizont. Eventually, the schizont bursts open, releasing the merozoites into the surrounding host environment. Some merozoites continue the sporogony cycle, entering new host cells and continuing the asexual cycle of replication though initiation of a subsequent round of merogony. Typically, multiple cycles of merogony occur. Other merozoites enter the gamogony cycle.
Gamogony: This portion of the life cycle involves sexual reproduction. Some merozoites enter a new host cell and form male or female gametes through meiosis. Following the creation of gametes, fertilization may occur, in which a microgamete enters a macrogamete, forming a zygote.
Sporogony: Following sexual reproduction, the zygote undergoes a period of asexual replication, culminating in the production of sporozoites, the invasive form of the parasite.
The most prominent morphological feature of this phylum is their possession of an apicoplast and an apical organelle. The apicoplast is a non-photosynthetic plastid derived from a chloroplast, which contains it's own DNA. It is thought to be an algal symbiont, and while it's exact function is unknown, it is essential to the organisms survival. This makes it a target for researchers looking to combat diseases like malaria. The apical organelle is used for entry into the host cell, and is composed of microtubules, a secretory body, and polar rings. Once inside the host cell, the parasite is able to convert the cell to it's own purposes.