108 Mr. J. Gray. 



movement of the ciliurn is the mechanical response. The cilia on adjacent 

 cells of the lateral epithelium beat in a definite sequence. If, however, 



A B C D E 



Fig. 4. — Diagram showing successive stages in the stroke of a cilium on the velum of a 

 gastropod larva (after Williams). A, position of rest : B, position at end of 

 recovery beat ; C and D, stages during effective beat ; E, end of effective beat. 



individual cells are separated experimental] y, they continue to exhibit active 

 and prolonged movement (Gray, 12). All attempts to detect the operation of 

 nervous elements in the epithelium, or in the cells themselves, have failed. 

 It may be concluded, therefore, that these ciliated cells provide an example 

 of an automatically contractile tissue. The cells are comparable to cardiac 

 muscle cells ; each cell is capable of independent movement, although, under 

 normal circumstances, there is a definite co-ordination between adjacent cells. 



When a piece of living Mytilus gill is teased in sea-water under the 

 microscope, portions of the cuticnlar layer with attached cilia are often 

 stripped away from the cells themselves. Such cilia are invariably motion- 

 less. It seems certain, then, that an essential part of the mechanism lies in 

 the cell itself ; as long as there is a small portion of normal protoplasm at 



B A 



Fig. 5. — Diagram of cilia of Plexirobrachia. A is the position of rest ; B is the end of the 

 effective stroke, whose direction is shown by the arrows. 



the distal end of the cell near the base of the cilium, the latter continues 

 to move.* 



* E. S. Lillie observed that detached but active cilia from the larva of Poljgordius 

 possessed a knob-like expansion at their proximal ends. 



