74 CONVERSATIONS ON 



this stone case has to be pretty nearly smooth, 

 so that the worm can drag it along on the bot- 

 tom of the spring or pond (for it never comes 

 out of it), and you will see that the picking out 

 of the stones alone is no trifle. But besides 

 this, it has to fasten them together with mortar." 



" And can the worm really do this, Uncle 

 Philip ? Will not the water wash the mortar 

 all away T 



" It certainly would if it were like common 

 mortar. It was a long time that men lived 

 before they found out a mortar that would re- 

 main, and grow hard under water. When 

 they want to build a wall that is to be under 

 the water, they use a cement which is called 

 pozzolana; it is made of lava out of a vol- 

 cano, and is water-proof. Our caddis-worm 

 has a cement too, which is better than pozzo- 

 lana, and though it has been tried, it cannot 

 be melted or dissolved in water. Here is a 

 drawing of the stone nest of a caddis-worm." 



Stone Nest of Caddis- worm. 



"Uncle Philip, you said that sometimes 

 these worms built their nests of other things 



