Cilia 649 



anisms within the organism. Such hypotrichs as Uronychia and Euplotes 

 show, not one type, but several types of locomotory pattern.''^ A role of the 

 neuromotor system in control of locomotion in Euplotes has been indicated 

 from experimental destruction of parts of this system (Ch. 23). In a number of 

 metazoans both direct and indirect evidence has been advanced pointing to 

 nervous modification of ciliary activity. ^'■*' ^°' ^^' •*- 



The rows of ciliarv combs of ctenophores are controlled by impulses pass- 

 ing out over the paddle rows from the aboral sense organs.^- ^'^ The ciliary 

 locomotory systems of the turbellarian, Stenostomiim, and of the ciliated 

 larvae of many molluscs and annelids are known to be under the control of 

 the nervous system.** The cilia may be active or motionless or, in the case of 

 Stenostomum, reversed. A freshly isolated piece of ciliated lip of the snail 

 Physa shows no ciliary activity unless a nerve fiber innervating this region 

 is stimulated.^*"' On the other hand, Alectrion, whose locomotion is largely 

 dependent on pedal ciliary activity, can inhibit these cilia. ^- If the foot is ex- 

 cised, there is first cessation of ciliarv beat, which is, however, graduallv re- 

 sumed again a short time after its removal, and then the beat continues 

 unabated for the remainder of the life of the ciliated epithelium. 



The cilia of the frog's mouth have been found to be accelerated greatly 

 and the amplitude of the beat increased after electrical stimulation of cer- 

 tain cranial nerve fibers."^ There is a latent period of about a second be- 

 tween beginning of stimulation and the response. The influence of the ner- 

 vous stimulation persists for as long as 10 minutes. It is not yet clearly shown 

 whether these fibers are of ^mpathetic or parasympathetic character, al- 

 though the bulk of the evidence appears to suggest their belonging to the 

 facialis group. Acetylcholine has been reported to increase ciliary activity. 

 In further support of normal nervous control of the cilia of the frog's mouth 

 is the observation that these cilia are normally quiescent;^'*' ''^ they com- 

 mence to beat in response to such stimuli as the addition of foreign par- 

 ticles. After these particles are swept away the ciliary beat again ceases. 



Influence of Environmental Factors on Ciliary Activity. Probably the 

 two most important environmental factors normally influencing ciliary activ- 

 ity are temperature and hydrogen ion concentration. The maximum rate of 

 ciliary activity occurs at about 34° C. CMytilus gill) and 35° C. (ciliated 

 epitheUum of the frog's mouth).--- -^' ^^ The Qio varies with the tempera- 

 ture range for which it is determined. For Mytilus gill cilia it ranges from 

 about 3 at the lower temperatures to slightly less than 2 at the high end of 

 the physiological range. Above a certain critical temperature there is a rapid 

 drop in rate. It seems reasonable to expect that the maximum temperature 

 for activity of ciliated epithelia would be higher for animals normally 

 found at higher temperatures than for animals living at lower temperatures. 



Hydrogen ion concentration, which varies considerably in nature, has a 

 more profound influence on the rate of ciliary activity than concentration of 

 any other ion.**- -'^- ^^- ^^' '''' Those acids like carbonic acid which penetrate 

 cells more rapidly have a greater effect than those which penetrate less rap- 

 idly.-^ Increase in hydrogen ion concentration can cause cilia to become 

 completely quiescent. 



A biological significance of this response of cilia to increased acidity is 

 seen in the fact that in an ordinary bivalve mollusc whose valves have been 



