iv PHYLUM CCELENTERATA 197 



many cases the same genera are found in widely separated 

 parts of the world. They are, however, larger and of more varied 

 form and colour in tropical regions, for instance on coral-reefs. The 

 largest reef-anemone, Discosoma, found also in the Mediterranean, 

 attains a diameter of 2 feet. Most members of the order are 

 littoral, living either between tide-marks or at slight depths, but a 

 few are pelagic, and several species have been dredged from depths 

 of from 10 to 2,900 fathoms. 



The Madreporaria, taken as a whole, have also a wide distribu- 

 tion ; but the number of forms in temperate regions is small, and 

 the majority including the whole of what are called reef-building 

 Corals are confined to the tropical parts of the Atlantic, Indian, 

 and Pacific Oceans, flourishing only where the lowest winter tem- 

 perature does not sink below 68 F. (20 C.). Thus their northern- 

 most limits are the Bermudas in the Atlantic, and Southern Japan 

 in the Pacific ; their southernmost limits, Rio and St. Helena in 

 the Atlantic, Queensland and Easter Island in the Pacific : in other 

 words, they extend to about 30 on each side of the equator. 

 Moreover, they have a curiously limited bathymetrical distribu- 

 tion, flourishing only from high-water mark down to a depth of 

 about 20 fathoms, but not lower. 



Many of the Pacific Islands are formed entirely of coral rock, 

 others are fringed with reefs of the same, and the whole east coast 

 of Northern Queensland is bounded, for a distance of 1,250 miles, 

 by the Great Barrier Reef, a line of coral rock more or less parallel 

 to and at a distance of from 10 to 90 miles from the land. 

 Such reefs consist of gigantic masses of coral rock fringed by living 

 coral, the latter growing upon a basis of dead coral, the interstices 

 of which have been filled up with ddbris of various kinds, so as to 

 convert the whole into a dense limestone. 



The Antipatharia, and many of the Alcyonaria, such as the Gor- 

 gonacea and Pennatulacea, have also a world- wide distribution, 

 and, even in temperate regions, both Black Corals and Sea-fans may 

 attain a great size : the members of both these groups, as well as 

 the Sea-pens, are found at moderate depths. The Red Coral is found 

 only in the Mediterranean, at a depth of 10 to 30 fathoms. Tubi- 

 pora and Heliopora have the same distribution as the reef-building 

 Corals. 



From the palseontological point of view, corals are of great im- 

 portance : they are known in the fossil condition from the Silurian 

 epoch upwards, and in many formations occur in vast quantities, 

 forming what are called coral limestones. The majority of fossil 

 forms are referable to existing families, but in the Paleozoic era 

 the dominant group was the Eugosa, the affinities of which are still 

 very obscure. The corallites are usually bilaterally symmetrical, 

 the septa are arranged in multiples of four, and the cup presents 

 on one side a pit, the fossula, where the septa are greatly reduced. 



