THE CORAL-WORM.
Captain Johnson told him there was nothing of the kind in that particular spot, but that a great deal of coral was to be found in the tropical waters of the Far East. "The best coral," said he, "comes from the Mediterranean; other parts of the world produce it in much larger quantities, but it is not generally fine enough to be wrought into jewellery, like that from the northern coast of Africa. Can you tell me what coral is?"
CUP-CORAL AND BRAIN-CORAL.
Frank answered that coral was a substance produced by a small insect which works under the water, and produces a substance somewhat resembling stone. There are many varieties of it, and the work of the coral insect is usually in the form of branches—like a small tree without leaves. There are also formations known as cup-coral and brain-coral, on account of their shape and general appearance.
Fred said he had read somewhere that in the Pacific Ocean there were islands of solid coral; and there were also reefs surrounding islands like great walls. Some of these walls were hundreds of miles in extent, and kept ships from approaching the land.
"Can you tell me what an atoll is?" said the captain, with a smile.
The boys had both heard of an atoll, but at the moment they were unable to describe it. So the captain came to their relief, and explained it to them.
"An atoll," said he, "is a circular island or reef, with an opening on one side, with water that is usually deep enough for the largest ships to enter. The strip of land or coral is a few hundred yards wide, and often covered with palm and other trees; and there are sometimes hundreds of atolls in a single group. They vary in size from half a mile to forty or fifty miles in diameter, and the lake or lagoon inside is from one to four hundred feet in depth. Ships may sail around in these lagoons, and they often abound in fish of many varieties. The contrast between the rough ocean outside and the calm lake within is very impressive, and will never be forgotten by one who has observed it."