ACTINOZOA. 191 



they are gracefully branched, and the whole 

 surface blooms with coral polyps in place of leaves 



of and flowers. Shrubbery, tufts of rushes, beds of 



ill pinks, and feathery mosses, are most exactly 



imitated. Many species spread out in broad 

 leaves or folia, and resemble some large-leaved 

 plant just unfolding ; when alive, the surface of 

 each leaf is covered with polyp flowers. The 

 cactus, the lichen clinging to the rock, and the 

 fungus in all its varieties, have their numerous 

 representatives. Besides these forms imitating 

 vegetation, there are gracefully modelled vases, 

 some of which are three or four feet in diameter, 



of , made up of a net-work of branches and branchlets 



and sprigs of flowers. There are also solid coral 



a hemispheres like domes among the vases and 



shrubbery, occasionally ten or even twenty feet 

 in diameter, whose symmetrical surface is gor- 

 geously decked with polyp-stars of purple and 

 emerald green." 



Under such aspects appear the living organisms 

 whose combined efforts have mainly constructed 

 those reefs and islands of Coral origin which now 

 lie scattered far and wide over the surface of the 

 tropical ocean. Three principal forms under which 

 such reefs occur have been distinguished by the 

 names of Fringing-reefs, Barrier-reefs, and Atolls. 



Fringing-reefs skirt the shores of favourably 

 situated lands, towards which, at a gentle slope, 

 they incline, ending abruptly seawards, where 

 soundings reveal a depth of from 20 to 30 fathoms. 

 The surface of the reef is covered at high water, 

 and forms a nearly level platform, from a few feet 

 to more than a mile in breadth, according to the 

 degree of inclination which the land presents. Its 



