36 THE OYSTER. 



long, and so fine that they are invisible under a low 

 magnifying power. They project from the sides of the 

 ridges, over the furrows between them, and therefore 

 overhang the water pores in the bottoms of the furrows. 



In a fragment cut from a fresh gill, each one of these 

 hairs is constantly swaying back and forth, with a 

 motion like that of an oar in rowing, quick and strong 

 one way, and slower the other way. They all move in 

 time, but they do not keep stroke, for each one comes 

 to rest an instant before the one on one side of it, and an 

 instant after the one on the other side. So that waves 

 of motion are continually running from one end of 

 each ridge to the other, like the waves which you have 

 seen running over a field of ripe grain, as each stalk 

 bends before the wind and then recovers. 



What would happen if a boat's crew were to row 

 with all their strength, with the boat tied to a wharf? 

 As they could not pull the boat through the water, 

 they would push the water past the boat. This is 

 exactly what the cilia do. They set up a current in 

 the water. Each one is so small that its individual 

 effect is inconceivably minute, but the innumerable 

 multitude causes a vigorous circulation, and each one 

 is set in such a position that it drives the water before 

 it from the gill-chamber into one of the water pores, 

 and so into one of the water tubes inside the gill ; 

 and as these are filled they overflow into the cloaca 

 and fill that. If the mantle were closed, all the water 

 would soon be pumped out of the gill-chamber into 

 the cloaca, but you remember that an oyster at rest 

 always has the mantle open. As fast as the gill-chamber 

 is emptied by the cilia, fresh water streams in from 



