The axis of a cilium or flagellum consists of microtubles arranged in a nine-fold radial symmetry, and is called the axoneme. It consists of 11 longitudinal fibrils, 2 central and 9 peripheral (9+2 arrangement). The two central microtubles are similar in structure to cytoplasmic microtubles. Each microtuble is a hollow cylinder composed of 13 longitudinal protofilaments. The 9 pheripheral fibrils are doublet microtubles, appearing like a figure of 8 in transverse sections. Each consists of two microtubles or subfibres attached length wise, and an incomplete tubule forming arms. The inner tubule of the doublet is called subfibre A and the outer tuble subfibre B.