. Introduction to zoology; a guide to the study of animals, for the use of secondary schools;. Atoll isa further step, in which a small island, formerly sur-rounded by a barrier reef, has disappeared, leaving a cir-cular reef surrounding a body of water (Fig. 205).Exactly how the central land disappears, whether by sub-sidence of the sea floor as the reef grows up or by beingwashed away, is still it matter of dispute. Budding and the Formation of Colonies. The Cnidariaare one of three groups of animals which have the habit of 218 ZOOLOGY forming colonies by budding, somewhat after the fashiono
. Introduction to zoology; a guide to the study of animals, for the use of secondary schools;. Atoll isa further step, in which a small island, formerly sur-rounded by a barrier reef, has disappeared, leaving a cir-cular reef surrounding a body of water (Fig. 205).Exactly how the central land disappears, whether by sub-sidence of the sea floor as the reef grows up or by beingwashed away, is still it matter of dispute. Budding and the Formation of Colonies. The Cnidariaare one of three groups of animals which have the habit of 218 ZOOLOGY forming colonies by budding, somewhat after the fashionof plants. The other groups are the Bryozoa, or sea-mats, and the Tunicata, or sea-squirts. In all casesthe buds arise from a definite part of the parent body anddevelop into a definite form, often exactly like that pro-duced from the egg. When the buds remain attached tothe parent, a compound individual or colony is colonies differ greatly in form. Thus among hy-droids we have colonies which produce runners, from whichalone, and not from other hydranths, new hydranths FIG. 205. —Atoll iii Fiji Islands (Nanuku Levu). The large circle of whitemade by breakers indicates the position of the coral reef. A small bit ofland still remains in the interior lagoon. Photo, by Dr. W. McM. Wood-worth. From A. Agassiz, Coral Reefs of Fiji. In another case (< )belia), one hydroid buds from the sideof another and rises beyond it, continuing the main stemof the colony. Since its descendants do the same, the stalkis made up of successive generations of hydranths. Some-times the hydranths are placed close together and oppo-site, like the leaves of Arbor vitte (Sertularia, Fig. 197).Again, there may be a main stalk composed of one hy-clranth and a series of lateral branches in one plane, mak-ing a fan-like arrangement of the colony. Or the lateral £5 *~ branches may arise in any plane, producing a bushy colony. THE HYDRA AND ITS ALLIES 219 The variety in the form of the
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