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. Galaxea Seriatopora Favia Porites Coeloria Stylophora (another form) Siderastrea Pavonia Favia Pavonia Pocillopora Pocillopora Porites Hydnopora (a common kind) _Plate XXVI_ [Illustration: Fig. 58. Stony corals of 13 species belonging to 9 genera; generic names only given] _Plate XXVII_ [Illustration: Fig. 59. _Dendrophyllia_, a simple colonial coral with distinct polyp cups]