The Mechanism of Ciliary Movement. 



105 



refractive and possess a considerable degree of elasticity. In this respect 

 Engelmann (8) expressed the view that : " Tons les organes vibratils sont 

 resistants, tres flexibles, et dans nne large mesure parfaitement elastiques." 

 The cilia of Mytilus are entirely independent of any control by the animal 

 and are in constant motion during the life of the cells. 



Lateral Cilia. 



On the sides of each gill filament are three rows of rectangular cells, each 

 bearing a brush of cilia. These are the lateral cilia (see figs. 1 and 2). The 



Fig. 1. — Lateral view of gill filament of Mytilus (modified from Orton). (a) Terminal 

 cilia, (b) frontal cilia, (c) latero-frontal cilia, (d) lateral cilia, (e) vertical water 

 current set up by lateral cilia, (/) direction of water current from frontal cilia. 



effective stroke causes a strong current of water to flow on to the gill surface 

 at right angles to it (see fig. 2). All the cilia arising from a single cell beat in 

 the same phase, as do also the cilia of the three cells comprising each vertical 

 row. 



The most distinctive feature of the lateral cilia is their marked rhythm. 

 The cilia on adjacent cells beat in succession, so that a continuous wave 

 passes along the whole line of lateral cells from one end of the filament to 

 the other ; the wave passes in opposite directions on the two sides of the 

 filament. This metachronial rhythm provides an interesting example of 

 co-ordinated movement which is not associated with any visible nervous 

 elements. Isolated individual cells from the lateral epithelium continue to 

 exhibit active movement. 



Latero-frontal Cilia. 



It will be seen from fig. 2 that on reaching the surface of the gill the water 

 columns set in motion by the lateral cilia meet the large latero-frontal cilia. 

 During one phase of the beat these cilia rapidly pass from the form of 

 straight rods to that of curved hooks, the point being directed towards the 

 free surface of the filament. The cilia then flatten out more slowly; the 



