234 UTILIZATION OF MINUTE LIFE. 



contains, and with which, it is supplied in great 

 measure by the polypes themselves, dissolves the 

 more ancient portions of their calcareous structure, 

 thus keeping a constant supply of carbonate of lime 

 at their disposal in the water. 



In the South Sea Islands, the madrepore struc- 

 tures are occasionally employed as building stone ; 

 they are known as coral-rock. 



Madrepora was formerly imported into this 

 country for medicinal purposes, under the name of 

 white coral. It is capable of receiving very fine 

 polish, and can then be made, as coral, into orna- 

 ments of every description.* 



* For many extremely interesting and novel details concerning 

 fresh-water polypes, bryozoa and infusoria, see Henry J. Slack's in- 

 genious little work entitled " Marvels of Pond Life." 



