river Shannon extends over one hundred and fifty square miles; and one-tenth of the whole island, it is estimated, is covered with peat, called by the Irish, turf. It is constantly accumulating, with a rate of increase affected by the amount of moisture and other circumstances; in Europe its increase is estimated at seven feet in thirty years. It is confined to the colder regions of the earth, since the heat of the torrid zone causes very rapid decomposition of organic matter. The process by which it is converted into coal has been, in some instances, observed. In some peat bogs large trees have been found erect; its antiseptic power over vegetable and animal substances is remarkable, preserving them from decay even for centuries. Peat swamps sometimes burst their barriers and deluge the surrounding country with black mud. 71. The floods of large rivers carry down immense quantities of timber, which meeting obstructions accumulate in rafts, or pass on to the delta, or the ocean. On the Mississippi and Red rivers, rafts have been formed several miles long, bearing soil and growing trees. The delta of the Mississippi contains many layers of wood undergoing the slow process of conversion into coal; but much of the driftwood passes out to sea, and is conveyed by marine currents to far distant coasts, or becoming water-logged, sinks to the bottom of the ocean. The Icelander is supplied with wood for fuel and building boats, by the ocean, which brings the drift-wood of the Mississippi and the rivers of Central America to the coasts of his island. The vegetable growth of arctic climates is stunted and slow, while that of the tropics is gigantic and rapid. 72. The most efficient organic agency in modifying the crust of the earth, is exerted by the most minute and insig nificant members of the Animal Kingdom-the coral Zoophytes and animalcules. 73. The animals which produce coral are very simple, resembling plants both in their figures and colors. Until the last century they were described as marine plants and flowers. They differ from plants in having distinct mouths and cavities to receive and digest food, and sensibility to pleasure and pain. They have no system of vessels for cirArbiculation, no glands, no distinction of sex, and no senses but those of touch and taste. Their texture is not that of jelly, but of flesh. They vary in size from a minute fraction 1219 peremt of an inch, to eighteen inches in diameter. They live tally or moreither solitary, or in masses of hundreds of thousands. Individuals are called polyps; the whole animal structure, whether simple or compound, is termed a Zoophyte. Coral is not a collection of cells in which the polyps may conceal themselves, but an internal skeleton: nor do they exhibit any instinct or industry in forming it. It results from vital processes in their system, which they no more control, than do the more highly organized animals the formation of their bones. The species are perpetuated by eggs and buds. The mode of budding is very similar to the budding of plants. A bud swells and bursts on the side or extremity of the parent, acquires tentacles and visceral cavity, and produces other buds and eggs. Polyps may also be multiplied by artificial section, each part having the power of reconstructing a complete animal. Every Zoophyte, however large or numerous the colony, commenced as a single polyp; successive budding may have produced myriads of polyps, which eat and digest separately, but all aid in the growth of the common mass. An injury to one of them is felt by the surrounding ones, but not al ways through the whole mass. Some polyps may be turned inside outward with no apparent injury, and the head of one polyp may be engrafted on the body of another. While the process of budding is advancing at the surface, death is occurring in the central and lower parts of the coral, which, when dead, serve only to support the external living part. 74. Numerous genera and species of these Zoophytes, have been described by Naturalists. A few of them will serve as illustrations of their general appearance. Fig. 31, presents three branches of the Caryophyllia, the polyps of which are of a bright green color, and reside in the radiating chambers. Fig. 32. Fig. 31. Caryophyllia. The Meandrina, or brainstone coral, so called from its resemblance to the convulutions of the brain of animals, as seen ellepearee Fig. 34. 2 ance. derives its name from its radiated or star-like appearThe polyp is represented in Figure 34, with its tentacles extended. It moves these tentacula, or arms, which are arranged about its head, with great rapidity in taking its food. 1. Star Coral. 2. The polyp magnified. borili in cabinets, and as mantel ornaments, The Flustra, Figure 37, is a deli- Madrepore. cate coral, often attached to sea weeds and shells thrown upon the shore. With a microscope, its polyps, if examined in the water, may be seen expanded, and retracted in their cells. 75. Most corals are white, even when the animals secreting them are highly colored. But the Corallium Rubrum-red or precious coral-is of a brilliant red color, while the investing animal is blue. It is obtained from the Mediterranean and Red seas, and is extensively used for ornaments. This species is shown in Fig. 38. Fig. 37. The Flustra. 1. Its cells. 2. The expanded polyp. 3. The polyp in its cell. An interesting variety of coral is the Tubipora, or organ-pipe coral, (Fig. 39,) composed of parallel tubes, with transverse plates indicating successive gene- found growing ocean several feet Fig. 38. in circumference, and its rich carmine red presents a bril 76. It is in the extensive coral-reefs, that the Zoophytes evince the power of organic agency in modifying the surface of the earth. The great reef along the line of the Illustrations 218,219.220, 232 on this au |