LAMP-SHELLS AND THEIR ALLIES 21 



come together. He has two tubes, or siphons, as 

 they are called, which he pushes up through the bur- 

 row to the surface of the sand or mud, and then 

 opens them out in the clear water above. Then he 

 starts his pump, which is a double-action affair, and 

 the work begins. Water is sucked down one pipe 

 and forced up the other, and with the water come 

 organic particles for food and dissolved air for 

 breath. His wonderful gills absorb the latter and 

 gather uj:) the former, which they pass on to the 

 mouth that is waiting to receive the nourishment. 



When the tide goes out and the sand is left bare, 

 our happy clam has just to wait, that is all. But if 

 you walk along over his hole he may become startled 

 by your footstep and suddenly pull down his siphons 

 into his shell. As they are full of water, the result 

 will be a little fountain which you will see spouting 

 up for an instant and then disappearing. If you 

 have a hoe or a shovel you may now dig the poor 

 fellow out of his revealed burrow, and his fancied 

 security will prove vain; but if he is a large clam 

 his burrow may be too deep to be easily explored, 

 and if he is lively he may dig too. 



If you pick up a dead shell and look inside one 

 of the valves you may see that it is marked some- 

 what like Figure 57, on a later page, except the 

 letters will not be there to guide you. The line p. 1. 

 is called the pallial line, because the pallium, or 

 mantle, or skin of the animal is attached to the 

 shell along this line. If the creature lives deep in the 

 mud and has long siphons there will be quite a notch 

 at p. s., which stands for pallial sinus. A sinus is 



