SHELLS. 33 



shelter in the deep of the ocean ; some * you 

 have learnt, anchor themselves to rocks and 

 thus bid defiance to dangers. But in spite of 

 all these, and many more equally beautiful con- 

 trivances, a breach is often effected in the habi- 

 tation of the mollusca. 



Child. And then the poor animals 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 expe- 

 rience 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 matter exuding from that part of 

 the animal which you had uncovered, and form- 

 ing 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 acces- 

 sions 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 



* Pinna. 

 D 



