MARINE BOTANY. 97 



spot which it occupied during life, and serves a new gene- 

 ration as the foundation on which it continues building. As 

 a general rule, all the lower strata of the larger Polypodous 

 aggregates are dead masses. Thus, the larger hemispherical 

 domes of the Astrese are covered with a living layer, which 

 is only half an inch thick ; and, in some of the Porites of 

 equal dimensions, the entire mass is found to be lifeless, 

 except a thin external crust of about one-sixth of an inch 

 in thickness. 



"We are amazed at the size of the Pyramids and primaeval 

 temples which a long-vanished race piled up on the shores 

 of the Nile ; but what are the colossal structures of the 

 Pharaohs when compared with the mighty walls which are 

 erected by the small weak zoophytes ? 



According to Darwin, to whom we owe the talented ex- 

 planation of the strange forms the Coral-reefs offer, these 

 animal edifices are naturally divided into three classes, 

 while their physiological mode of structure always remains 

 the same. 



One description of reef is immediately connected with the 

 continental or island shores (shore reefs, fringing reefs) ; to 

 this variety belong all the Coral-banks of the Red Sea, 

 which Ehrenberg and Hemprich investigated during eight 

 months. 



A second variety forms, at a greater distance from land, 

 a wall, which either runs along the coasts (barrier reefs), or 

 encloses a central island (encircling reef). Among these is 

 the great Barrier-reef which lies opposite the north-eastern 

 coast of Australia. According to Flinders, it has a length 

 of nearly a thousand miles, and runs parallel with the coast 

 at a distance of twenty to thirty miles, which, in some parts, 

 extends to fifty or seventy. 



The huge arm of the sea, formed in this manner, has an 

 average depth of ten to twenty fathoms, which at one end, 

 however, increases to sixty, while the open sea beyond the 

 reef is of unfathomable depth a very short distance off. The 



