has part has_part has part cilium A specialized eukaryotic organelle that consists of a filiform extrusion of the cell surface and of some cytoplasmic parts. Each cilium is largely bounded by an extrusion of the cytoplasmic (plasma) membrane, and contains a regular longitudinal array of microtubules, anchored to a basal body. eukaryotic flagellum flagellum microtubule-based flagellum primary cilium intraciliary transport particle cell projection organelle