182 



ANIMAL BIOLOGY 



the umbo, in ever widening concentric rings, are the lines of growth 

 of the animal, which indicate younger stages in its development. 



Let us now pull up a mussel and lay it on a sandy bottom. In a 

 few moments the shell will open somewhat and from one end will 

 project a pinkish body, which may finally extend some distance. 



This organ is the foot. If 

 we watch long enough, we 

 may see the mussel use the 

 foot to push itself over the 

 surface of the sand or it 

 may burrow into the sand, 

 and finally come to occupy 

 a position like that in 

 which we found it. 



FIG. 130. Trichina in Muscle. (Leuckart.) 



exhalent siphon 



Now if one is patient, and the animal feels at home, it will be pos- 

 sible to see the method of eating and breathing. At the end oppo- 

 site the foot there may slightly project from the shell a fringed and 

 somewhat tubular-shaped structure. Let us place a little finely 

 powdered carmine in the water above the 

 opening. As the carmine slowly sinks 

 and comes opposite the tube, the particles 

 will suddenly be drawn into the tube. This 

 shows that water is being sucked into the 

 tube, and it brings with it oxygen and 

 any food that may be near, such as mi- 

 croscopic plants and animals. 



To learn any more about the feeding 

 and breathing of the mussel it will be neces- 

 sary to open the shell. Let us take an- 

 other mollusk and pry open the valves. 

 We shall soon find that this is not easy to do. The reason will be 

 evident after studying Figure 132. 



The valves are held together by strong muscles. So we pry the 

 valves open a little with a heavy knife and then slip another 

 sharp knife in close to the valve, where we meet an obstruction 

 toward one end. When we have cut this, the valve opens at that 



foot 



FIG. 131. Mussel bur- 

 rowing in sand. 



