The electron microscopic study of a cilium or the flagellum show that they are covered with plasma membrane. Their core called axonema which is about 2-10 pm long in cilia and about 150 pm long in flagella. The axoneme is composed of 11 microtubules. Out of these two microtubules are single and lie at the centre with a gap between. They are called central singlets. They are connected together by a double bridge. They are surrounded by a common central sheath. The remaining 09 microtubules are double and lie in a ring around the central microtubules. The one tubule of the each peripheral doublet is connected with central sheath by a radial spoke. Thus, there are nine radial spokes that connect the peripheral doublets with central sheath (the covering of centrally located single microtubules or singlets). This arrangement of axonemal microtubules is also called 9+2 arrangement. In non motile or primary cilia, the two central single microtubules are absent so the central bundle consists of 9 + 0 arrangement of microtubules. In prokaryotic cells, the flagella are filamentous protein structures composed of flagellin. The peripheral doublets are also interconnected by linkers. Both the cilium and flagellum emerge from centriole like structure called basal bodies. Chemical Structure: The microtubules (single or double) are made up of tubulin protein. The arm of one microtubule (of each peripheral doublet) contain a protein dynein which converts the chemical energy stored in ATP to mechanical work. The linkers of interlinking the doublets are made up of nexin. The radial spokes and central sheath are formed of protein of unknown nature. Types of Flagella: 1. Whiplash or Acronematic flagellum: When flagellar membrane has no hairs on its surface, it is called whiplash or acronematic flagellum. 2. Tinsel or Pantonematic flagellum: When flagellar membrane has stiff hairs (called flimmer filaments) on its surface, it is called tinsel or flimmergeissel flagellum. Types of Cilia: 1. Non - motile or Primary cilia: These are found in nearly every cell in all mammals and as the name suggests these do not beat. They can be found in human sensory organs such as eye and the nose. 2. Motile cilia: These are found on the surface of cells and they beat in a rhythmic manner. They can be found in the lining of trachea (windpipe), where they sweep mucus and dirt out of the lungs. In female mammals, the beating of cilia in the fallopian tubes moves the ovum from the ovary to the uterus.