650 Comparative Animal Physiology 



held tightly closed for two or three hours out of water there is a high enough 

 concentration of CO:., to inhibit ciliary activity. Inasmuch as O^ consump- 

 tion of ciliated tissue is a direct function of the rate of ciliary activity, this 

 influence of CO2 is obviously adaptive. The minimum hydrogen ion con- 

 centration necessary to produce inhibition of gill cilia varies with different 

 molluscan species. For example, it is lower in Mya, which customarily dwells 

 in well-aerated water, than in Ostrea or Mytilus. 



Another very striking instance of an apparently adaptive variation in the 

 minimal hydrogen ion concentration essential for ciliary inhibition has been 

 reported for the cilia of the different portions of the digestive tract of MyaJ'^ 

 There is a deBnite correlation between the normal pH of each region of the 

 digestive tract and the pH at which the cilia of that region are brought to 

 rest. 



Other cations and also anions are also known to influence ciHary activ- 

 j,.y 20. 21, 2.S, 24. 25, 29, 33 jj^ general, these ions are relatively constant in any 

 given environment in nature. The effects of altering their concentrations 

 and ratios appear to be general physiological ones, quite comparable to their 

 action on all cells; and, as with other cells, cation content of the medium 

 has more influence on ciliary beat than has anion content. 



Theories of Ciliary Movement. Little or nothing is known of the actual 

 mechanism of ciliary motion. Historically, theories have fallen into two gen- 

 eral categories: (1) those that assumed the moving force to occur in the cell 

 body proper, with the cilia acting only passively, and (2) those that assumed 

 that the moving force occurred in the cilium itself, the cilia therefore mov- 

 ing as a result of their own contractile powers. The original proponent of 

 the view of cilia behaving as passive bodies was Schafer,"-* who considered 

 the cilia to be hollow elastic structures with differences in the degree of 

 elasticity of various portions of their walls. The cilia were believed to beat 

 as a result of a rhythmic surging and ebbing of fluid of the cell body into 

 and out of these cellular extensions. 



Much more compatible with our modern theories of the mechanism of act 

 ion of contractile elements in general is the second type of theory, namely, 

 that the cilium itself is an actively contractile element with only one side 

 of the ciHum contracting or with the two sides contracting alternately. This 

 view, first clearly advanced by Heidenhain-'* in 1911, has since acquired 

 many other advocates. Supporting this point of view are numerous observa- 

 tions, including the fact that the waves passing along a flagellum may often 

 show no reduction in amplitude as thev pass from base to tip. Such a reduc- 

 tion would be expected were kinetic energy generated only in the cell body 

 proper at the flagellar base. In species such as Peranema, the undulatory 

 activity may be restricted exclusively to the tip region of the flagellum, the 

 remainder meanwhile apparently remaining quiescent. Again, the use of 

 such methods as transmission of polarized light. X-ray diffraction, and elec- 

 tron micrography indicates the molecular organization of a cilium to resem- 

 ble in a striking manner that of muscle fibrils which are definitely known to 

 possess inherent contractility. Both exhibit a fibrillar structure obviously 

 due to an orientation of elongated molecules or micelles. 



There is as yet no agreement of opinion as to which portion of the cilium, 

 the axial filament or the sheath, is the actively contractile portion. In sup- 



