440 CILIARY MOVEMENT. 



993. In looking at the apparatus, by which the various 

 bodily movements are effected, that are concerned either in 

 obtaining food, or in changing the place of the entire body, we 

 observe a considerable diversity in the Radiated classes. In the 

 lowest, the whole tissue appears equally contractile ; whilst, in 

 the highest, a distinct muscular structure exists, in which this 

 contractility specially resides. There is another very remarkable 

 structure, however, which is widely diffused through the group ; 

 and which enables very active movements to be performed by 

 animals, in which no distinct muscular structure can be detected. 

 This is termed the ciliary apparatus; and, as its extensive 

 diffusion through almost the whole animal kingdom, and great 

 importance in the economy, have only of late been recognised, it 

 will be desirable that we should pause here for a short time, to 

 examine its nature in some detail. 



994. The organs termed cilia are little hair-like filaments, 

 covering the surface and fringing the edges of various membranes 

 both external and internal, which are in contact with fluid ; and 

 in this fluid they produce, by their vibrations, currents which 

 may serve various important purposes in the economy of the 

 animal. In the active and free-moving Infusorial Animalcules, 

 the cilia on the exterior of the body are the principal, if not the 

 only organs of locomotion : in the Polypes, fixed to a particular 

 situation, and unable to go in search of food, the currents which 

 they produce in the surrounding element bring it within reach of 

 their tentacula or arms ; and in all animals modified for respira- 

 tion in water, from those simple structures in which no particular 

 division of the surface seems appropriated to this function, to 

 Fishes, and the larvae of Batrachia (481), their movements 

 appear to have an important relation with it, in constantly 

 renewing the stratum of water in the neighbourhood of the 

 aerating surface. Cilia are even found on the mucous membrane 

 lining the trachea, and ramifying air-passages of the higher Verte- 

 brata; and their use appears there to be to convey the secretions 

 and foreign particles, if such should be present, along the surface. 

 They have also been observed in the upper part of the alimentary 

 canal of Reptiles, throughout its whole extent in the Mollusca, 



