SHELLS. 33 



have learnt, anchor themselves to rocks and thus 

 bid defiance to dangers. But in spite of all these, 

 and many more equally beautiful contrivances, a 

 breach is often effected in the habitation of the 

 mollusk. 



Child. And then the poor animal must perish. 



Teacher. This is by no means inevitably the 

 case, for they are gifted with the power of re- 

 pairing their shells. 



Child. How can they manage this ? 



Teacher. By a discharge of the same cal- 

 careous matter, of which they at first constructed 

 their shells. You might by your own experience 

 prove this to be true ; if you procure in the spring 

 some common snails, break off a part of their 

 shells, then giving them a supply of food, place 

 them under a glass that you may watch their 

 operations ; you would observe first a frothy mat- 

 ter exuding from that part of the animal which 

 you had uncovered, and forming a film ; in a 

 short time a second discharge would raise the 

 first to the level of the external surface of the 

 shell, and by degrees fresh accessions would give 

 the new part the thickness and substance of the 

 old shell. 



Child. It would be very amusing to watch 

 this experiment. 



Teacher. But do you not admire the instinct 

 that so wonderfully directs these little animals in 

 their self-preservation ? 



Child. It is very surprising certainly, that 



D 



