The Apicomplexa are a unique group because all members are parasitic. Members of this group lack any visible means of locomotion (most are intracellular) and all have complex life cycles. The common feature of all members is the presence of an apical complex in one or more stages of the life cycle. Although the exact components of the apical complex (visible only at the ultrastructural level) vary among members, its function, presumably penetration, is important.
The apical complex is found in the sporozoite and merozoite stages of all genera of malaria, piroplasms, coccidia (and related organisms) and Pneumocystis carnii. The apical complex consists of one or two electron dense polar rings at the anterior end of the cell, a conoid (seen in some coccidia) that lies within the polar ring, and two or more rhoptries located within the polar ring extending posteriorly from the plasma membrane. Subpellicular microtubules extend from the polar ring parallel to the long axis of the cell (except in the genus Babesia) presumably functioning as support for the cell. Micronemes lie parallel to the rhoptries and often merge with them at the apex of the cell. Rhoptries and micronemes probably are secretory organelles and facilitate penetration into host cell.