38 THE OYSTER. 



walls are filled with blood-vessels in which the blood 

 comes into close contact with the water. 



The way in which the current of fresh water is kept 

 up to bathe the gills continually with a new supply is- 

 more complicated. 



When one of the ridges on the surface of the gill is 

 examined with a high power of the microscope, it is 

 found to be fringed on each side by a row of fine 

 hairs, Plate III, Fig. 2, c, c, each one less than ^ inch 

 long, and so fine that a good microscope must be 

 used to see them. 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 hairs do. They set up a current in 

 the water. Each one is so small that its individual 



