Half Hours with the Lower Animals / Protozoans, Sponges, Corals, Shells, Insects, and Crustaceans

by Charles Frederick Holder

Fig. 37.—Coral head, with many polyps (Astræa).

In the surf, where it piled in upon the reef, grew a beautiful form known as leaf coral, spreading out like the horns of the moose in great leaflike shapes. This crept near the ground, and was surrounded by its cousins, the Gorgonias, in lavender and yellow. The whole presented a beautiful appearance when seen from above through a water glass or glass-bottomed boat.

In most of these corals the branches were covered with the small cells of the coral animal, made up of thousands of individual polyps. Others again had very minute cells, yet the entire head might weigh a thousand pounds. Another large head is called brain coral, as the animals are arranged in deep trenches or convolutions. In the star coral (Astræa, Fig. 37) the polyps resemble stars and are much larger than those on other corals.

Occasionally I have found a branch of coral on which there was, perhaps, a bunch of eight cells, each half an inch across, the group resembling a bunch of flowers. These were generally in the deeper parts of the lagoon, where the water was fifteen or more feet deep, and therefore out of reach of the coral tongs. I would, therefore, dive down for it, the coral being distinctly visible in these clear and limpid waters. This rose coral, as we called it, was the work of a few polyps. Another kind was very delicate, the polyps being almost invisible. It was called pepper coral, as when tasted it burned the tongue violently. Still another, which grew in heads a foot or two across, had a peculiar habit of floating when free of animal matter. Large heads, when tossed from the beach where they had drifted, went sailing away like boats.

Still another coral has cells at short intervals up the branch; another is cup-shaped with a single polyp. One of the most remarkable corals (Fig. 38) has the cells of the polyps arranged after the fashion of a pipe organ, from which the coral takes its name, while the polyp itself, when expanded, resembles a daisy. Formerly corals were supposed to be confined to the warm waters of the tropics, but this is true only of the reef builders, which require a temperature not lower than 63°, and are rarely, if ever, found at a greater depth than about 180 feet. Single polyp corals, like Fungia, are found at great depths in the ocean, and certain corals grow in the Santa Catalina Channel on the Pacific coast. In the Atlantic, as far north as Long Island Sound, where the water is often icy cold, is found the beautiful Astrangia, a coral in which the polyps are pure white and about five one-hundredths of an inch in length.

Fig. 38.—Organ-pipe coral (Tubipora): A, cell tubes; B, polyp expanded.

In a general way we have passed in review some of the typical corals, and may now glance at their manner of growth. If we cut one of the cells of a coral across, we shall have a figure similar to that shown in Figure 39. The white radiating partitions are coral, the black spaces are rooms, which correspond to the little apartments in the anemone. The coral develops by eggs and by budding, just as in the case of its cousin, the anemone. The eggs, after enjoying a free-swimming life for a while, settle upon the bottom and begin to secrete lime. They do not build up a house as the mythical "coral insect" is described as doing, but secrete it much as any animal secretes its bones or shell. As the water flows through the animal it is enabled to secrete the lime dissolved in the water. If we could watch every step of the growing process, we should first see a little platform of lime attached to the stone or object upon which the young coral animal has dropped, then a little edge or rim which increases in size daily. Out from this rim shoot the partitions, as shown in Figure 39. It will be observed that they do not meet and join, but leave a place in the center for the stomach. Finally, the cell is completely formed, and we have a perfect cup of lime, a coral cell in which is ensconced the anemonelike coral polyp. Its color is an olive brown, and when the polyp is expanded its little tentacles resemble the petals of a flower. With these it catches food, which it eats in very much the same way as do the anemones. If this cup is a branch coral, soon a bud appears upon the side, and a new cup or cell takes shape. Then another is added, and we see the coral enlarging, branching out either by budding or simply dividing until a large branch is the result.

Fig. 39.—Sections of a coral cell.