THE FRESH-WATER MUSSEL. 



97 



dttctor, and the door which leads out of the back-room 

 is the upper tube, or dorsal siphon. This little back- 

 room, or dorsal chamber, which lies behind the inner 

 gills, is also called the cloacal chamber, because it is 

 the place where the intestinal tube empties. Cloaca 

 means a sewer, or drain. There is a very old one, 

 which empties itself into the Tiber, at Rome, called 

 the Cloaca Maxima, for it is so large. When the 

 water is low, you can see this great opening in the 

 bank, and it looks like the mouth of a cave. Now, 

 what is the use of the gills ? Perhaps, if we try to see 



Fig, 05. 



V-shaped Gill-Pocket.— The Aeeow-Heads point the Direction of Currents. 



how they are made, we can find out what they do and 

 what they are made for. Each gill is a little pocket, 

 shaped like the letter V (Fig. 95). If you look at the 

 gills, you will see a number of strips, or lines, running 



up and down, and if you put a very small piece under 



. 5 



