Cilia 641 



tion is locomotion commonly show the characteristic of being under the con- 

 trol of the coordinative mechansim of the organism, enabling the animal to 

 orient its movement, or cease moving. This control of locomotor ciliation ap- 

 pears to be superimposed upon a fundamental ciliary automaticity.-" 



In those instances in which the surrounding medium, rather than the cili- 

 ated cells, is caused to move, cilia may be 'subserving one or more of numer- 

 ous functions, such as feeding, circulation, cleansing, respiration, and the 

 movements of materials within ducts. The action of cilia establishes feeding 

 currents for many ciliate protozoans and for sessile or sluggish species in a 

 wide variety of animal groups. The feeding currents of rotifers are easily ob- 

 served. The ciliary activity of the tentacles and oral regions of numerous 

 sea anemones and corals serves to sweep non-nutritious particles away from 

 the mouth and off the tips of the tentacles into the surrounding sea wa- 

 jgj. 5.5, 79 However, in the presence of food, the tentacles which are, in re- 

 sponse to the food, tipped toward the oral cavity pass these materials into 

 the gastrovascular cavity. In Metridium marginatuvi, among other species, it 

 has been reported that the direction of the effective beat of the cilia in the 

 stomodaeum is reversed under the stimulus of food.'^^' ''' The unstimulated 

 cilia normally beat outward, but in the presence of food such as crab flesh 

 they beat inward. KCl produces an action similar to that produced by food.'^^ 

 In Actinoloha there are longitudinal grooves in the stomodaeum, the cilia 

 on the ridges beating outward and those in the grooves beating inward. The 

 degree of muscular contraction of this organ would therefore be expected to 

 influence strongly the direction of the dominant currents created.^"* 



Probably nowhere in the animal kingdom are ciliary mechanisms more in- 

 tricately developed than in the ciliary filter-feeders, principally the lamelli- 

 branchs,^' ^^' '^ certain gastropods,^^ and protochordates.''^ Here the sys- 

 tems are so organized that cilia set up feeding currents, filter out suspended 

 particles, collect them into specific ciliary tracts, and convey them to the 

 oral opening of the digestive tract. Along the route special ciliary mechan- 

 isms for sorting out the particles on the basis of size, discarding the larger 

 ones, are not uncommon. 



Materials are transported within the digestive systems of numerous in- 

 vertebrates by ciliary action. Definite courses of circulation through the 

 coelenteron canal systems of Aurelia~- and Pleurohrachia}'^ have been lucidly 

 described. Ciliary circulation is an important means of the movement of 

 food in the digestive system of some echinoderms,^^' numerous molluscs,^ 

 and manv other organisms. 



A cleansing role of cilia appears evident on the basis of the normal direc- 

 tion of the effective beat in sea anemones. In many starfish cilia beat from 

 mouth toward anus over the entire surface, sweeping away debris. The action 

 of the cilia of the epithelium of the frog's mouth and that of the ciUa of the 

 respiratory epithelium of mammals appear also to be largely examples of 

 this role. 



Clear demonstration that cilia play a role in facilitating respiratory ex- 

 change, apart from the functions which have already been described, is 

 usually impossible. Any constant exchange of the medium in intimate contact 

 with the ciliated epithelium would accomplish such a function in at least 

 some degree. 



