THE FRESH-WATER MUSSEL. 



107 



tubes, or capillaries, it gives up to the body the pure 

 oxygen and other good matters it contains, and receives 

 the poisonous carbonic acid and waste part, and hence 

 when it leaves the capillaries it is bad (Fig. 100), or 

 what is called venous blood. When the blood leaves 

 the fine hair-tubes, or capillaries, it enters vessels 

 called veins, and they carry the blood to the kidney, 

 where some of the bad matters are strained off. It 

 continues its journey, and now enters the gills, and is 

 spread out in a net-work of fine capillaries over the 

 lattice-work of the gill-pockets (Fig. 96). There the 

 blood gives off its carbonic acid and receives oxygen. 

 Now it is pure again, and is carried by vessels and 

 poured into the two side-rooms, * or auricles, of the 

 heart (Fig. 100). The auricles close or draw themselves 

 together, and force the blood through little folds, or 

 valves, into the middle-room, or ventricle. The 

 valves do not allow the blood to return to the auricles. 

 The ventricle draws itself together and sends the 

 blood into the tubes called arteries, to continue the 

 same course, and, as the blood goes in this circle through 

 the body, its course is called the circulation of the Mood. 

 But why do we not see the blood in these rooms pass- 

 ing from the auricles into the ventricle, for the walls 

 are transparent? If the blood were red like yours, you 

 could see it moving along, but it has none of the red 



