126 EXERCISE ON CLAM, BY MERCY J. DAVIS. 



order that it might still be protected, its house grew also. Each line shows 

 what was once the edge of the shell. The spaces between the lines on the 

 shells show the different periods of growth. You will notice that one end 

 of the house is more rounded than the other. This end is downward. You 

 can easily see how this rounded part keeps the shell from sinking too deeply 

 into the mud. 



You know how water wears away the rocks against which it washes. 

 Sometimes it wears away shells and makes them crumble. Sea -water some- 

 times even dissolves them, leaving only a fine powder. It would not do for 

 the clam's house to crumble away, so the outside of it is covered with a thin 

 skin. In this way it is protected from the rude action of the sea- water. Sup- 

 pose we look within the shells. First, we must force them apart. This is 

 not easily done. We find that they are fastened quite firmly together by two 

 muscles, for the clam needs to be well protected. We must cut these mus- 

 cles to see the interior of the house. Just look at the inner walls of our 

 clam's house ! How smooth and polished ! How much prettier than the out- 

 side ! Perhaps here is a lesson for us. We do not find all the pretty things 

 until we ourselves hunt for them. What seems rough at first sight, may be 

 found to have some beauty after all. What makes the inside look so much 

 more shiny and pearly than the outside ? The body of the clam is too delicate 

 to come in contact with the hard shell. It is therefore surrounded by a 

 smooth, thin mantle. The edges oi the mantle are thickened. They may 

 be seen between the edges of the shells when they are in their natural po- 

 sition around the clam. The smooth, pearly surface is made by the mantle. 



We have learned a few facts about the house of the clam. Now let us 

 try to learn something about the little friend who lives in this shell house. 

 The first thing that we see is the wrinkled, black portion which extends be- 

 yond the shell. This is called the siphon. Do you find two openings in 

 the end of the siphon ? They are surrounded by ray-like projections. These 

 openings are the mouths of tubes which extend into the clam's body. Later 

 on, we will tell you about their use. Let us remove one of the shells as we 

 did before from the other clam. We can now see all of the clam. I can imagine 

 your saying, " What a queer body the clam has ! It has no head ! It has 

 no legs! " But let us see what we can find. The four thin, delicate bodies 

 which float upward as we put the clam into a dish of water are the gills. 

 These have the sams use in their bodies as our lungs have in our bodies. 

 The clam cannot breathe the oxygen from the free air as we do. It breathes 

 oxygen from the air which is in the water. The water containing the air 

 passes through the large opening which we saw in the siphon, into a tube, 

 through which it enters the body. It then passes over the gills, and around 

 the body, and out through the smaller opening in the siphon. In this way 

 the clam gets the oxygen that it needs. 



