Cilia are fine hair like membrane covered protoplasmic outgrowths that occur on the free surface of cells and take part in producing a current in fluid medium for passage of materials and locomotion. Cilia are smaller but numerous. Flagella are longer but fewer. Cilia occur in group ciliate of Protista, flame cells of worms, larva bodies of many invertebrates, epithelium of respiratory tract, renal tubules etc. Flagella occur in group flagellata of protozoan protistans, euglenoids, dinoflagellates, choanocytes of sponges, gastrodermal cells of coelenterates, sperms of animals, bryophytes and pteridophytes, zoospores and gametes of several algae. A cilium or flagellum consists of three parts: basal body, basal plate and shaft.
Functions of Cilia and Flagella
1. Cilia and flagella help in locomotion of ciliated and flagellate organisms (e.g., Paramecium, Chlamydomonas) and structures (e.g., zoospores, spermatozoa and other gametes).
2. In land animals, the cilia of the respiratory tract help in eliminating dust particles in the incoming air.
3. Internal transport of several organs is performed by cilia, e.g., passage of eggs in oviduct, passage of eggs in oviduct, passage of excretory substances in the kidneys, etc.
4. Cilia and flagella are able to perceive a number of sensations.
5. Tips of cilia and flagella secrete agglutinins to help in conjugation of gametes.
6. Cilia of some protists fuse to form an undulating membrane.
7. They help in capturing food in some protists and animals.
8. Flagella of gastrodermal cells circulate food in gastrovascular cavity.
9. Flagella of choanocytes or collar cells create water current in the canal system of sponges.
10. Cilia create a current for movement of food and ingestion.
11. Cilia and flagella create a current in water for quick dispersal of carbon dioxide and replenishment of oxygen.
12. In land animals, cilia of nasal cavity, trachea and bronchioles move the mucus trapped dust particles towards pharynx for elimination.
13. Nephric filtrate moves in the urinary tubules with the help of cilia.
14. The ovum released from ovary passes into infundibulum and fallopian tube due to cililary action. Newly formed sperms pass through vasa efferentia in a similar fashion.
15. Certain organisms have ciliated larvae. The latter bring about dispersal of species.