down into roundish gray lumps that attract neither the eye nor the appetite of any marauder. [Illustration: CORALS Tree coral (_Dendrophyllia nigrescens_). Tuft coral (_Lophophyllia prolifera_). Fan coral (_Euphyllia pavonia_). Cup corals and skeleton (_Carophyllia smithii_).] The coral polyps differ from anemones only in details of structure that we need not consider, except to note the striking difference that here the base and the radiating partitions instead of being membranous secrete a firm skeleton either of lime or of the horny material termed chitin. The flesh overflows the walls, folding down from the top, so that the skeleton becomes really internal, although naked at the broad base. Some of the tropical stony corals are like big anemones, several inches across; and it is only when they infold all their richly colored tentacles and become a dull and shapeless lump that their stony cup is revealed. These are solitary, and form loosely lying corals, like that called the "mushroom." New ones are produced by the parent throwing off buds which for a time remain attached by a stalk, but finally fall off and settle down to grow--a process that may go on for a score of years. In the case of the huge coral masses called madrepores the buds remain attached to the parent. If they spread out naturally, W. Saville Kent explains, they build up by accumulation the large rounded masses known as "brain" corals and "star" corals, which are most numerous on coastline reefs, or form the base of the outer barrier reef. On the other hand, where the budding is terminal, or oblique, branching, treelike growths result in "staghorn" and similar forms. The coral animals do not alone construct the reefs. Stony hydroids (millepores), shells of all sorts of mollusks, limy sea mosses (Bryozoa), animalcules and diatoms and various algæ stiffened or cased with lime or flint, and blown sand, contribute to build them up, especially when they near the surface of the sea.