A spiral cytoskeletal structure located at the apical end of the apical complex in some apicomplexan parasites. Fibers form a left-handed spiral, and are comprised of tubulin protofilaments organized in a ribbon-like structure that differs from the conventional tubular structure characteristic of microtubules. [ http://www.ncbi.nlm.nih.gov/pubmed/16518471 http://www.ncbi.nlm.nih.gov/pubmed/11901169 ]