8o 



CORALS AND CORAL ISLANDS. 



surface, and are always without any prominent calicles, there 

 being only very minute rounded punctures over the surface, 

 from which the animals show themselves. The cells in the 

 corallum are divided parallel to the surface by very thin plates 

 or tables, as in the Pocilliporae and Favosites ; and they were 

 formerly classed, therefore, with other tabulate corals. 



S. P. Sharpies found the coral of M. alciconiis to consist 

 of 97*46 per cent, of carbonate of lime, 0*27 of phosphate 

 of lime, and 2-54 of water and organic matters. The 

 Millepores are very abundant corals, and eminently so in 

 the West India seas, contributing largely to the material of 

 the reef^. 



The Hydroids were long considered polyps. But they have 

 been found to give origin to Medusae, or jelly-fishes, and it 

 is now proved that they are only 

 an intermediate stage in the deve- 

 lopment of Medusse, between the 

 embryo state and that of the adult 

 or Medusa state. The Millepores 

 afford, therefore, examples of coral- 

 making by species of the class of 

 Acalephs. Many of these Medusae 

 and their Hydroids will be found 

 illustrated in the admirable work 

 of Alexander and Mrs. L. Agassiz, 

 entitled " Sea-Side Studies," — an 

 excellent companion for all who 

 take pleasure in sea-shore rambles. 

 Another genus of corals referred to the Millepora group 

 occurs in the East Indies, the species of which is remarkable 

 for having within an indigo-blue colour; it is called Helio- 

 pora cmridca, the generic name, from the Greek for sun, 

 alluding to the minute roiuid polyp-cells. This and the true 

 Milleporas are coral-reef species. A few allied species occur 

 in colder waters, and for these the genus Pliobothrus has been 

 instituted ; one species has been described by De Pourtales, 

 from the deep waters off the Florida reef. The ancient 



ANIMALS OF MILLEPORA ALCI- 

 CORNIS, MUCH ENLARGED. 



