THE RAZOR 53 



you were to look at it again, you would find that 

 it was almost triangular. A few hours later still 

 it would be round again, just as it was when it 

 first hatched out of the egg. And during this 

 time of its life it has a strong fleshy "foot," like 

 that of a snail, so that if it becomes tired of 

 swimming it can settle down and crawl about 

 on the surface of the rocks. 



Have you ever been through the Thames 

 tunnel? If you have, you will be interested to 

 know that it is made just like a shipworm's 

 burrow, for a kind of boring instrument, called 

 a " shield," was made, which enabled the work- 

 men to line the walls with masonry as fast as 

 the earth was cut away. In this way the walls 

 were prevented from falling in, and water from 

 the river above was kept from breaking through 

 the roof and flooding the tunnel. And Brunei, 

 the great engineer who constructed the tunnel, 

 admitted that the idea had come to him one 

 day when he was examining the burrow of this 

 wonderful mollusc. 



PLATE XIX 

 THE RAZOR (i and 2) 



If you walk about very quietly, when the tide 

 is out, on the stretch of wet, sandy mud which 

 lies just above low-water mark, you may often 



