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Mr. J. Gray. 



flattening begins at the base and proceeds to the point. These cilia also exhibit 

 a certain degree of metachronism, but to a much less marked extent than 



Fig. 2. — Transverse vertical section of three gill filaments, showing the deflection of the 

 vertical current on to the frontal cilia, (a) Frontal cilia, (b) latero-frontal cilia, 

 (c) lateral cilia. 



the lateral cilia. The function of the latero-frontal cilia appears to be two- 

 fold : (i) they act as vanes which deflect the water currents on to the surface 

 of the filaments ; (2) they keep the individual filaments apart, so as to give 

 free play to the lateral cilia. 



Frontal Cilia. 



The whole of the flat frontal surface of the gill is covered by the frontal 

 cilia, whose effective beat is parallel to this surface and directed towards its 

 free edge (see fig. 1).* 



When the movement is very greatly reduced in speed by the addition of 

 gum arabic to sea-water, it is seen that, during the effective beat, the cilium 

 behaves as a more or less rigid rod, which moves forward on a pivot at its 

 base. During the recovery stroke, however, the cilium assumes entirely 

 different properties — it is drawn back as a limp non-elastic body in which a 

 stress is set up which starts at its base and is transmitted to the free end, 



* At the end of the filaments the frontal cilia are modified so as to deflect the current 

 towards the food grove and towards the oral end of the gill. These modified frontal 

 cilia are very obvious, and will be referred to as the terminal cilia. 



