104. THE WORLD VOT TE SEA. 
The polypiers sometimes occupy immense areas. They rise 
above the surface of the waves, forming reefs which surround 
islands, join them to each other and to their neighbouring continents, 
thus filling up the depths of the ocean. As long ago as 1702 
& 
A POLYPIER HYDRARIA PRODUCING A MEDUSA. 
Strachan observed that the coral reefs were formed in this manner. 
His account was received with credulity, and it was not until 1780 
that Forster, the learned companion of Captain Cook, established 
beyond dispute that most of the islands of the South Sea owed 
their existence to the excessive multiplication and compact agglo- 
meration of the polypiers. Since that day, this has been con- 
firmed by a great number of sailors, zoologists, and geologists. 
