Plate XVII. The movement of two types of cilia from 

 Mytiliis gill filaments. 



{a) Two cycles of beating of an abfrontal cilium. Note the 

 quick " flick " from the resting position at the start of the 

 movement to the upright position, and the slow return to the 

 gill surface. x 100. 



{b) One cycle of beat of two large frontal cilia. The two 

 cilia lie close together and beat more or less in unison. The 

 recovery phase of the beat takes considerably longer than the 

 effective phase. ; 100. 



Both sequences filmed at 24 frames per sec; prints have 

 been retouched to enhance contrast. 



