104 



BOYS AND GIRLS IN BIOLOGY. 



still hold the animal with the straight dorsal border 

 upward, you can see where the two halves of the man- 

 tle are joined together. Near the front you will notice 

 a thin, transparent part of the mantle, and if you look 

 through this spot you can see something beating. This 



Fig. 99. 



Aorta. — - (a 



Intestine 



Large Blood-ves- 

 sel, or Aorta. 



Side-Room. 



Auricle. 



Middle-Room, or Ventricle. 



Mussel's Heart. 



thin place forms the upper part of a sac — a cavity filled 

 with fluid, and in this sac, surrounded by this fluid, lies 

 the heart. This accounts for the beating. The sac, or 

 bag, is called the pericardium, because it surrounds the 



