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Updated: May 14, 2025
In the same way certain of the higher Radiates undergo all their transformations, from the Polyp phase of growth to that of Acaleph or Echinoderm, after birth; while others pass rapidly through the lower phases of their existence within the egg, and are born in their final condition, when all their intermediate changes have been completed.
The animal which constructs them is of the polyp tribe, and so small that it can be seen only under the higher powers of the microscope. It multiplies by means of buds like those of a tree, the individuals all combining to form a composite stony mass, which is called a polypidom. A number of such polypidoms growing close together form a coral reef. See woodcuts.
A little higher up, in fresh-water polyp, the internal sac which digests and the outer skin which serves to envelop it can, if absolutely necessary, change their functions; if you turn the animal inside out like a glove it continues to live; its skin, become internal, fulfills the office of a stomach; its stomach, become external, fulfills the office of an envelope.
A dull fish would show up amid such rich colours, so it is easy to know why Coral fish wear such fine clothes. Many of them spend all their time among the Coral, their food being the living tips of the Coral "branches," which they nip off with fine, sharp teeth. Others have teeth like millstones, fit for crushing the hard Coral, and eating the fleshy body of the polyp within.
Plenty of people still talk about the coral 'insect. It never occurs to them to call an anemone an 'insect, but they don't know that the coral polyp is more like an anemone than anything else." "But an anemone is a soft flabby thing that waves a lot of jelly-like fingers about in the water."
It is made of the shells of little animals, called the polyp, which live in the sea." "So you see that the firm ground under foot is made of rock, some of which has been ground up over and over again. But there is something else besides rock that makes the world" "Water," said Donald promptly as he looked down upon the river.
It is by its soft parts alone those parts which are seen only in its living state, and when the animal is fully open that its Acalephian character can be perceived, and this accounts for its being so long accepted as a Polyp, when studied in the dry Coral stock. Nothing could exceed my astonishment when for the first time I saw such an animal fully expanded, and found it to be a true Acaleph.
In structure each tiny polyp is little more than a fleshy sac whose mouth is surrounded with petal-like tentacles, or feelers. From the sea water the polyps secrete calcium carbonate and build it up into the stony framework which supports their colonies. Boring mollusks, worms, and sponges perforate and honeycomb this framework even while its surface is covered with myriads of living polyps.
Gone was the storming, impetuous rebel; his body was ruled solely by the precise and insatiable brain of the research scientist. "He's great, that kid Perce! When I see him, I'm going to kiss him on both cheeks. He's got enough dope on them to hang them higher than Franklin's kite, and we'll nail those jaspers to the cross or I'm a polyp!
The freshwater polyp Hydra, like the foregoing illustrations, is one whose structure has already been discussed in the earlier chapters, but now we may use it for an analysis of another series of biological phenomena. Its sac-like body consists of two cell-layers; the outer one is concerned primarily with offense and defense, while the inner layer is made up of digesting or nutritive elements.
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