i904-i9°5-] Recent Views regarding Coral Reefs. 173 



first of these the tentacles and the radiate parts of the animal 

 run in sixes or multiples of six — whence these corals are re- 

 ferred to as the Hexacoralla. In an extinct group, abundantly 

 represented by the corals found in rocks of Carboniferous age, 

 there is more or less of what one may term right- and left-ness, 

 and a front can be made out as distinct from the back. In 

 these corals there is more or less tendency for the radiated 

 parts to run in fours. So these are often referred to as the 

 Tetracoralla. In another group, represented at the present 

 day by the Sea Fans, the Precious Coral, the Organ-Pipe 

 Coral, the Dead -men's Fingers, and some others less well 

 known, the tentacles are eight in number, neither more nor 

 less, and the tentacles are fringed. These corals are therefore 

 referred to as the Octocoralla. Lastly, there is a group of 

 corals of lower grade than these, in which the structure of 

 the polyp resembles that of the Hydra more than the Sea- 

 Anemone. These are called the Hydrocorallinse. They are 

 well represented by the existing Millepores. The first-named 

 group and the last are those most concerned in building up 

 coral reefs. The Millepores are of great interest from both a 

 geological and a biological point of view. 



Corals are all confined to marine habitats. So far as their 

 relation to depth of water is concerned, it is well to remember 

 that the several genera and species may range to various 

 depths. Some are confined to the depth of quite a few fathoms, 

 while others range to the deepest parts yet explored by the 

 dredge. So far, again, as the temperature of the water is 

 concerned, there are some corals which can live in water of 

 almost icy coldness, while others cannot endure a temperature 

 that falls below 68° Fahrenheit. 



Again, so far as the mode of growth of corals is concerned, 

 there is considerable diversity of habit. Some coral animals, 

 like Sea-Anemones, never bud or branch, but remain simple 

 or non-compound structures throughout their lives. Others 

 — and these include the reef-building forms — branch to an 

 almost indefinite extent, just as a tree buds and branches out 

 of the parent plant. Both of these multiply by ova, regarding 

 which more presently. 



Adult coral animals are fixtures ; but their ova are sent 

 forth into the sea- water, where they hatch into fry endowed 



