POLYPES. 229 



some of the rarer marine Algce along with these 

 magnificent polypes. 



It is to the genus Madrepora that most of the 

 so-called " coral-reefs" are owed. Every one knows 

 how dangerous these reefs prove to navigators, and 

 what an extensive part they play in the consti- 

 tution of the earth's crust. Their colours are almost 

 invariably white or yellowish- white ; but there are 

 some which are completely yellow, red, or brown. 

 These Madrepora are extremely common in nature, 

 and abound near the islands of the South Sea, of 

 the Indian Sea, and especially near the Antilles. 

 Captain Cook tells us " that he could not sail 

 through certain straits which he had passed with 

 ease a few years previously, on account of the pro- 

 digious and rapid multiplication of these coral- 

 reefs." There is a barrier reef of madrepores that 

 runs along the whole of the eastern coast of 

 Australia. Captain Flinders endeavoured for four- 

 teen days to pass through it, and he found that he 

 had sailed more than five hundred miles before he 

 accomplished his purpose. Throughout the whole 

 range of Polynesian and Australian islands, there is 

 hardly a league of sea unoccupied by a t( coral-reef" 

 or a " coral-island." 



These reefs develop themselves in proximity to 

 the shores of continents and islands, or upon the 

 summits of submarine volcanic rocks. The latter 



