iv PHYLUM OELENTERATA 207 



to more than once. The corallum contains not only the apertures 

 for the polypes and siphonozooids, but also tubular cavities of 

 an intermediate size, in each of which is found a small chsetopod 

 Worm, belonging to the genus Leucodore. As the polypes are 

 frequently found retracted at a time when the Worms are protruded 

 from their holes in search of food, it is not surprising that the 

 latter should have been credited with the fabrication of the coral. 

 Trapezia, a genus of Crabs, always lives in interstices of a particular 

 species of Madrepore. 



The distribution of the Actiniaria is world-wide, and in 

 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, Bio 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. More- 

 over, they have a curiously limited bathymetrical distribution, 

 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 coat 

 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 debris of various kinds, so as to 

 convert the whole into a dense limestone. 



The Antipatharia and many of the Alcyonaria, such as the 

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

 and, even in temperate regions, 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, and at the 

 Cape Verde Islands : other species of Corallium occur on the coast 



