EFFECTOR MECHANISMS 367 



a series of triangular plates in close contact with one another but lacking a 

 common matrix. The cilia in each plate or membranella beat in unison. 



Ciliary Movement 



When observed under the microscope cilia appear to be moving at a 

 high velocity. Actually the linear velocity is rather low; Bidder estimated 

 the average velocity of the tips of flagella in sponges at 4 m/hour. The 

 velocity varies during a cycle, that of the effective stroke being about five 

 times the recovery stroke. The angular velocity, on the other hand, is of 

 a much higher order, since cilia can oscillate through half a circle at a 

 rate of 10-12 strokes/sec. 



Cilia and flagella have several forms of movement which, on analysis, 

 resolve themselves into three fundamentally different types. The beat of a 

 given cilium may conform to one of these types, or to some combination 

 thereof. The simplest type is pendular movement, in which the cilia vibrate 

 to and fro by flexing at the base. The forward effective stroke occurs more 

 rapidly than the return recovery stroke, but there is no difference in the 

 path traversed during the two phases of the cycle. This type of movement is 

 characteristic of larger compound cilia, e.g. those of heterotrichous ciliates. 



A second type of movement involves flexing of the cilium, the flexure 

 beginning at the tip and progressing towards the base. During recovery a 

 reverse process takes place, the cilium straightening out progressively 

 from base to tip. This type is exemplified by latero-frontal cilia of lamelli- 

 branch gills. The third type, characteristic of flagella, involves an undulatory 

 movement, in which a series of waves passes along the length of the flagel- 

 lum from base to tip. 



Combinations of these several types of movement are seen in various 

 cilia. Frontal cilia on the gill filaments of Mytilus, for example, show a 

 stiff pendular movement during the effective stroke, whereas the recovery 

 stroke is accomplished by progressive flexing of the limp cilium. Cilia 

 usually beat in one plane and act as paddles. Flagella, however, beat in a 

 spiral course and impart a rotatory movement which causes them to act 

 as screws or propellers. The flagellate Monas can move in several different 

 ways — forwards, backwards and laterally (Fig. 9.1). During fast forward 

 movement the flagellum sweeps in pendular fashion through an arc of 

 about 90 c (the effective stroke), whereas the preparatory stroke involves a 

 bending or flexure beginning at the base of the flagellum and progressing 

 towards the tip. When Monas moves backward the flagellum shows waves 

 passing from base to tip; in lateral movement the flagellum is flexed at 

 right angles and undulates: the waves may involve the whole length of the 

 flagellum, or be restricted to the tip (51, 74, 76, 77). 



Co-ordination and Control of Ciliary Activity 



Cilia appear to be inherently automatic organelles. Small pieces of 

 ciliated epithelium or fragments of ciliated protozoans continue to show 

 ciliary activity for some time after removal from the parent organism. 



