It would be of great interest to learn exactly how these humble creatures take lime from the water and produce shells of such marvelous beauty: to learn why one is of lime and others, like the Radiolarians, are of silica; why some live at the surface and are free swimmers, while others creep about in the ooze. When the deep-sea explorers first began to dredge, they found over vast areas a peculiar mud or ooze, and investigation showed that it was formed almost entirely of the shells of a certain minute shell maker, from which it was named the Globigerina ooze. Finding these vast banks or beds of mud at this depth is suggestive that the deepest seas may yet be filled by the dropping of this invisible rain; but as the average depth of the ocean is nearly if not quite three miles, many centuries must pass before this will be accomplished. [Illustration: FIG. 12.--Noctiluca. Highly magnified.] The marvelous phosphorescent light previously described came from a minute armored form known as Peridinium, but even this is exceeded by the glories of a little monad called Noctiluca (Fig. 12). It is a giant of the tribe, and is visible to the naked eye, being almost as large as the head of a pin, and resembling a currant in shape. It has a single hairlike organ or lash, supposed to be a locomotive organ, by which it whirls itself through the water. Of all the light givers of the sea this is the most common, some of its species being found in every sea, and where they are, it is necessary only to splash the water to cause a blaze of light to follow. A French naturalist placed on record the fact that so brilliant was the light occasioned by this minute organism in African waters that he read by their light standing on a beach where a heavy surf came pounding in upon the sand. The light of this little creature is remarkable not only for its vividness but for its many different tints. Now it is a fitful vivid green, again the water is a blaze of yellow light, or orange. At such times, when a ship is plowing along, the light is so brilliant that the sails and rigging are brilliantly illumined, casting weird shadows.