648 



Comparative Animal Physiology 



Studies on the progressive anesthetization of the ciliary mechanism have 

 shown that, in protozoans, the first thing to be lost is the power of ciliary 

 reversal. At a later stage metachronic coordination is lost, leaving the cilia 

 beating independently. Last of all, the cilia entirely cease their activity. 

 This observation has led to the hypothesis that ciliary beat, metachronism, 

 and reversal are controlled by three separate mechanisms. Reversal must be 

 governed by a mechanism of relatively rapid transmission affecting all cilia 

 of the organism nearly simultaneously and passing in all directions over the 

 animal. Transverse cuts passing deeply into those protozoans investigated in 

 this regard do not measurably interfere with the transmission of excitation in 

 response to stimuli which induce ciliary reversal. 



Fig. 246. Diagram of a ciliated epithelial cell of Anodonta showing regions where cuts 

 were made. Cut at A always resulted in cessation of ciliary beat. Cuts at B through F 

 permitted continuation of beat but resulted in more or less loss of normal coordination. 

 Cuts at G through J resulted in no apparent disturbance of normal ciliary activity. From 



Worley.'^ 



Support for the concept of the transmission of excitation in intracellular 

 ciliary coordination by a fibrillar system has come from microdissection 

 studies of the ciHated epithelial cells of clam gills'^^' '^^' ''^ (Fig. 246). Cuts 

 through the distal third of the cell containing the ciliary cone of intracellular 

 fibrils destroy ciliary coordination; cuts through any level of the proximal two- 

 thirds have no such effect. Only when transverse cuts are in the immediate 

 region of the basal granules themselves does even the uncoordinated ciliary 

 beat cease. Waves of ciliary metachronism pass smoothly over large sheets 

 of ciliated epithelial cells; the mechanism for this intercellular transmission 

 is unknown. 



Although ciliary beat appears in general to be the result of an automaticity 

 of the cilipry mechanism, yet there are numerous bits of evidence that cilia 

 in many cases are controlled in their ac'ivities by moie basic response mech- 



