INTRODUCTION. XI 



are thrown out in abundance, thicken into a con- 

 sistence round the mouth of the shell. Thus, as the 

 animal increases in bulk, it continues to produce 

 fresh materials, and enlarges its house till it has ac- 

 quired its full growth. 



The same process of the animal economy which 

 enables the snail to build a house, will also serve to 

 keep it in repair. If we break out a piece of the 

 shell, without wounding the snail, the skin beneath 

 will soon become covered with a viscid exudation, 

 which thickens and congeals by degrees. Twenty- 

 four hours after the operation, the repair will be 

 fairly begun, a thin layer will be visible, and this 

 will continue to thicken, till, in ten or twelve days, 

 the new piece will be nearly as thick and strong, as 

 the rest of the shell. A similar matter is transuded 

 also from the collar of the snail, when she wishes to 

 close the mouth of her shell against the approach 

 of winter. This thin calcareous operculum, or lid, 

 is very different from the permanent lids of several 

 other shells. It is not fixed to the head of the 

 animal, but is perfectly free from any part of the 

 body, though strongly adhering to the mouth of the 

 shell. It is easy to prove its shelly nature; for, 

 when plunged into diluted muriatic acid, its calca- 

 reous parts dissolve and leave merely a gelatinous 

 tissue behind. 



