THE FRESH-WATER MUSSEL. 



99 



which leads into the back-room, or dorsal chamber. 

 Now, what is the use of the water, and why should it 

 be kept in constant motion % The little blood-vessels in 

 the skin of the pockets are full of bad blood, which 

 comes to the gills to be made pure ; that is, to get 

 oxygen, just as your blood must go to your lungs to 

 get oxygen. Your lungs get oxygen from the air 

 which you breathe in, aad they give out the bad air, or 

 carbonic acid, which comes out with your breath. The 

 blood-vessels of the gills get their oxygen from the 

 air in the water. They take it up as the water rushes 

 over them, and give up the carbonic acid which they 

 do not need. So now you see why the lashes keep 

 moving ; the blood-vessels in the gills must have fresh 

 water continually, and the cilia are like so many mill- 

 ion little paddles, or fans, of a windmill, that make a 

 strong current inward through the gills, from the ven- 

 tral or front room, to the dorsal or back room. The 

 current of water rushes into the house of the mussel, by 

 the large ventral siphon, or the lower opening. You 

 know that this siphon is fringed by strong hairs, or 

 cilia (Fig. 91), so they bring in the water from the out- 

 side, and the hairs on the gills carry it on. The gills 

 are the mussel's lungs, but he never has consumption 

 or lung-disease, because he always supplies himself with 

 plenty of fresh water. We have seen how the water 



