450 CILIARY MOVEMENT. 



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. 



1086. 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 alimentary mat- 

 ters within reach of their tentacula or arms ; and in all animals 

 modified for respiration in water, from those simple structures in 

 which no particular division of the surface seems appropriated to 

 this function, to Fishes, and the larvaB of Batrachia ( 521), their 

 movements appear to have an important relation with it, in con- 

 stantly 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 aliment- 

 ary canal of Reptiles, throughout its whole extent in the Mol- 

 lusca, and in the stomach and its appendages in the Asterias, as 

 well as in many other situations. 



1087. The presence of cilia, when they are moving with 

 rapidity, can frequently be inferred only from the eddies which 

 they produce in the neighbouring fluid. Sometimes the return- 

 stroke, which is made more slowly, can be seen when the direct- 



