Cilia

Cilia, are minute vibratile hair-like processes, which are capable of rapid vibration backwards and forwards: they occur in most groups of animals. Their function is usually to create currents of water either for the collection of food, e.g. in the Rotifers and Brachiopoda, for respiration, as in the gills of Lamellibranchiate Mollusca, such as the mussel, or for the removal of excreta, as in the ciliated funnels of worms. In other cases they may serve for locomotion as in the case of Paramecium, the "Slipper animalcule." Their true nature is most easily seen in the Protozoa: they result from the attenuation of pseudopodia, the protoplasmic processes put forth by many of the lower members of that phylum; when these pseudopodia lose their direct locomotive function they may acquire the habit of merely vibrating in one plane; they then become much finer, and drawn out into cilia. One of the three great divisions of the Protozoa is known as the Ciliata (or Infusoria) owing to the importance of cilia to the members of this division; it is classified into orders according to the arrangement of the cilia.