Page:Popular Science Monthly Volume 42.djvu/534

 516 Still, the chalky deposits are not wanting in them, but they are microscopically small, are scattered, and rarely exhibit patterns or knobs. The radial structure is easy to recognize in many of them, but not in others, especially in the deep-sea forms which have only recently become known. But this is a modern variation. The ancient typical structure of all the spiny-skins is radial, and the number of rays is five or a multiple of five. When rays appear in other numerical relations (based on 1, 4, or 6), it may be traced back to a recent variation. While the mouth in the sea urchins and starfishes is on one of the broad sides of the somewhat flattened body—which for this reason is designated as the buccal, or, not very accurately, ventral region—in the sea cucumbers the body extends from the mouth to the other pole, and the animals are not flat, like their relatives, but lengthened out like worms. They, therefore, do not move on the mouth-surface. Thus they are transformed from a radial symmetrical structure into an apparently bilaterally symmetrical, right-and-left structure, but really equally lateral, and, superficially regarded, look like thick, plump worms. Around the mouth is a fringe of tentacles, shield-formed or greatly branched, which serve as organs of touch and groping, or perhaps for breathing. The size of the animals varies greatly; there are forms in the depths of the northern seas which measure but little more than a few centimetres, while tropical species living near the surface are two feet long and more. Their stupidity and slowness of motion correspond with the kind of food they live on. They fill themselves with sand and the detritus of crumbled corals; and, as they do not hunt for food, they need no eyes or organs for rapid motion. In those sediments of the sea are enough organic substances—products of decay, algæ, animals of the lowest species—to keep the slow metabolism in action by their motion. Such inert animals would soon fall a prey to the always hungry robbers of the sea if they had to depend on their skill and dexterity. But they seem to have other means of keeping their enemies away; possibly they have a bad taste to them, or their tough, leatherish skin causes them to appear undesirable morsels