. The popular natural history . Zoology. SEA-URCHIN. -581 of the Radiates, such as the sea-urchin, the whole body is encrusted with a chalky coat, while in others it is as soft and easily torn as if it were composed of mere structureless gelatine. The mode of walking, or rather creeping, which is practised by these bemgs, IS very mteresting, and may be easily seen by watching the proceed- mgs of a common star-fish when placed in a vessel of sea-water. At first it will be quite still, and lie as if dead, but by degrees the tips of the arms be seen to curve slightly, and then the creature s
. The popular natural history . Zoology. SEA-URCHIN. -581 of the Radiates, such as the sea-urchin, the whole body is encrusted with a chalky coat, while in others it is as soft and easily torn as if it were composed of mere structureless gelatine. The mode of walking, or rather creeping, which is practised by these bemgs, IS very mteresting, and may be easily seen by watching the proceed- mgs of a common star-fish when placed in a vessel of sea-water. At first it will be quite still, and lie as if dead, but by degrees the tips of the arms be seen to curve slightly, and then the creature slides forward without any perceptible means of locomotion. If, however, it be suddenly taken from the water and reversed, the mystery is at once solved, and the walking apparatus is seen to consist of a vast number of liny tentacles, each with a little round transparent head, and all moving slowly but continually from side to side, sometimes being thrust out to a considerable distance, and sometimes being withdrawn almost wholly within the shell. These are the " ambulacra," or vvalking apparatus, and are among the most extraordinary means of progres- sion in the animal kingdom. Each of these innumerable organs acts as a sucker, its soft head being applied to any hard substance, and adhering thereto with tolerable firmness, until the pressure is relaxed and the sucker released. The suckers continually move forward, seize upon the ground, draw the body gently along, and then search for a new hold. As there are nearly two thousand sucuers continually at work, some being protruded, others relaxed, and others still feeling for a holding-place, the progress of the creature is very, regular and gliding, and hardly seems to be produced by voluntary motion. We will now proceed to some examples of these curious beings. We first take a beautiful family of this order, called Echinidse, because they are covered with spines like the quills of the hedgehog. Popularly they are known
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
