Plate XXV, Figs. [55] and [56], represents such a form, which is in fact the only British stony coral[40]. But for differences in shape and proportion, the upper part of the organism is exactly similar to the sea-anemone shewn in [Fig. 52], but beneath it is a stony mass, the coral cup, secreted by the base of the polyp, a seat exactly adapted to its own shape. The empty cup, after removal of the anemone, is shewn as seen from above in [Fig. 57], to the right, and the curious radial plates of the same stony matter, so characteristic of all coral cups, are very plain. This polyp is comparatively large, measuring half an inch or more across.
This cup is not formed in the complicated way in which bone is made in the higher animals. The material is the cheapest possible, viz. limestone; this occurs in minute quantities in solution in all sea-water, and the coral polyp has the power of absorbing it[41] from the water and rendering it insoluble and stony in just those places where it is needed to form the kind of cup characteristic of the species. Another difference from bone is that the secretion is altogether outside the body of the animal; the cup is a mere dead structure from the first. One can imagine the animal as throwing down a limestone seat for itself, and as the seat thickens the polyp is raised more and more above the sea-bottom.
Plate XXVI
| Galaxea | Seriatopora | Favia |
| Porites (another form) | Stylophora | |
| Siderastrea | Coeloria | Favia |
| Pavonia | Pavonia | Pocillopora |
| Pocillopora | Porites (a common kind) | Hydnopora |
Fig. 58. Stony corals of 13 species belonging to 9 genera;
generic names only given
Plate XXVII
Fig. 59. Dendrophyllia, a simple colonial coral with distinct polyp cups
Imagine now the polyp to bud, as does Palythoa, and each bud to secrete its own cup, while the connecting branches also throw down the same limestone, so that the cups are connected on to one mass, and we have at once the formation of ordinary reef coral, of which perhaps the simplest possible case is the Dendrophyllia, figured on [Plate XXVII], where each polyp gives rise to but one bud, which gives out one other, so each branch is like a simple chain of polyps and their cups. The other corals figured are rather more complicated, since one polyp gives off many buds, and their branches are correspondingly more massive. In the hemispherical corals, the connecting branches are short, practically non-existent, and the polyps are crowded together, as a kind of skin, over the solid mass of limestone they have secreted. Certain species of coral form enormous colonies, containing hundreds of thousands of little polyps. I remember a certain part of the fringing reef of Zanzibar[42] over which the water was 6 feet or more deep. Being perfectly clear, and so favourable to coral growth, it was inhabited by a species of the genus Porites, which formed huge cylinders, the flat tops measuring 6 to 12 feet across, level with that of the lowest spring-tide, since it is impossible for the polyps to live above that level. So closely were these great cylinders planted in the water that it was easy, by striding and jumping from one to another, to cross the channel to the shallower part of the reef on the other side.