60 FIRST LESSONS IN ZOOLOGY. 



a spade in digging, it makes a hole, down which the clam 

 rapidly sinks. After digging its way down for nearly a 

 foot, out of harm's way (for the clam has many enemies), 

 it remains with its head end downwards, and its siphon 

 greatly extended, straight upward in its hole, which is 

 partly open, at least enough to allow the water to trickle 

 down (Fig. 59, A). 



In walking over mud flats we see little pits in the mud ; 

 these are the holes of the clam. Sometimes when we step 

 near the hole a jet of water shoots into the air. This is 

 spurted out by the clam. Alarmed by the approach of a 

 supposed enemy, the clam suddenly closes its shell, draws 

 in its siphon, and, as the latter contracts, ejects the water 

 from its shell. How does the clam know that any one is 

 coming ? It can hear. There is in the middle of the foot 

 a little yellowish-white ear, connected by a nerve with the 

 brain in the foot. Iferves run from the foot-brain to the 

 upper brain, and thence nerve-threads extend to the siphon, 

 and branch out in each tentacle ; so that when a noise is 

 heard the news is sent to the brain, and the latter repeats 

 the message to the muscles of the siphon and shell, so that 

 the valves close and the siphon is drawn in, while the clam 

 remains quiet until the disturbance is over. 



The clam breathes, digests its food, and circulates its 

 blood throughout its body in the same manner as the 

 oyster, which we will describe farther on. 



The clam has a very large family. Prom early in Septem- 

 ber to the middle of October, or about forty days, it lays 

 each year hundreds of thousands of eggs. Some of the eggs 

 are probably eaten by other animals, or become in some 

 way spoiled ; but the larger proportion hatch, and the little 

 clams swim about in the water, where most of them, how- 

 ever, are eaten by other animals, and few out of the origi- 

 nal numerous brood get to be full-grown clams. Hence, 

 though it has myriads of young, they are exposed to so 

 many dangers that few clams survive, and there are no 

 more clams now than there were hundreds of years ago. 



