Popular Science Monthly/Volume 12/April 1878/Living Corals
PERHAPS enough already has been written about corals and coral-builders, but certainly too little is really known about the habits and mode of growth of this interesting animal. I am fortunate enough to possess tine specimens of some three or four varieties of living, working coral "polyps," for such is the name by which the coral-builder is designated. These specimens are kept in pure sea-water, and I have studied their habits very carefully for years; they seem to be in perfect health, full of activity, and generally industrious, although I can hardly observe that they have added to their corallum during this period. A specimen of Astrangia which has been described by Dana and Elizabeth Agassiz, although it has not, perhaps, increased visibly in height, has enlarged in size by building from its outer edges, and numbers of young and smaller polyps have appeared. I have noticed that this budding and increase occur about October or November of each year.
Nothing can be more beautiful than a group of these corallets. Sometimes a mass of them of three or four inches across is dredged up at Wood's Hole, Massachusetts. The polyps rise high above the cells, and, with their long, slender, fleecy, silk-like tentacles swaying to and fro, remind one of a living bed of roses, or of a mossy bank covered with heavy frost.
I have seen the animal of a single corallet standing above the surface of its cell an inch in height and expanded to one-third of an inch in diameter, and yet many—I may safely say, most people—believe the "coral-insect" (not an insect at all) to be a microscopic or a very minute animal. Its long, snowy tentacles are covered all over with warts or dots, with a larger one at the tip of each. All these contain its weapons of defense, called lasso-cells, or capsule-threads, with which the animal captures its prey. The smaller Crustacea, coming in contact with these nettle-like arrows, are surely and suddenly disabled, and then with the longer tentacles are drawn inward to the mouth, which is situated in the centre of all these arms. This food passes into the stomach, which can be plainly seen through its glass-like walls, and must be digested before the animal retreats into its calicle. I have given them pieces of clams or oysters as large as half a pea, which they would seize with their tentacles and readily swallow. Another reason for this particular variety of coral-builder being so interesting to us is, that it is the only true coral-building polyp we have north of the Florida reefs, or nearer our homes than the Bermuda Islands.
Another variety, which belongs to the Oculina tribe, I have in the same healthful condition. This one assumes a branched or tree-like form, and is, if possible, more elegant and beautiful than the last. Its zoöthome, when fully expanded, seems to completely hide the whole corallum, and nothing is visible but one mass or tree of living, moving flowers.
This polyp does not elongate itself so much as does Astrangia, and its tentacles are much shorter; and, instead of being white, it is of an umber-like color, and covered all over with iridescent hues. Its septa, or radiated plates of the corallum, are the same as in Astrangia. The common mass of coral is of a more solid, stone-like nature; the epitheca, or outer coral layer, which the Astrangia has not, and the endotheca, or coral inside the cells, are also more compact and hard.