As to the cause of the light in the star-fishes, little is known. Quatrefages, after a careful examination of an Ophiuran, came to the conclusion that the light emitted was due to muscular contraction; observing it arising between the plates of the arms and not on the disk, where, however, it has been seen since his observations were made. Professor P. Martin Duncan found upon examining a specimen, brought from the icy sea of North Smith’s Sound, by Sir George Nares’s expedition, that it had a delicate mucous envelope, which, he thought, in the young covered the plates and bases of the spines. In this filmy covering, he suggests, may be found the seat of the illuminating power. CHAPTER V. SUBTERRANEAN LIGHT-GIVERS. In wandering through the fields in early morning, we often see little heaps of newly disturbed earth, and occasionally catch glimpses of reddish or pink bodies quickly withdrawing into little tunnels in the sod. These are the earthworms, considered the humblest of all animals; yet, as insignificant as they seem, they are among the most valuable aids to the agriculturist. We may appreciate this by selecting a field at random in a good producing country, making a section down through the earth for several feet, when, if carefully done, we shall find innumerable tunnels formed by the worms, leading here, there, and everywhere. In fact, the upper crust of the earth is an endless maze of streets, lanes, and avenues. A naturalist has even attempted to calculate the number of these little workers, and has come to the conclusion that they average one hundred thousand to the acre; and in especially rich ground in New Zealand it was estimated that there were three hundred and forty-eight thousand, four hundred and eighty in a single acre. This vast body of worms is continually at work, boring this way and that, coming to the surface during the night, and retreating to greater depths during the day; and it is at once evident that their tunnels constitute a system of irrigation and ventilation for the upper crust. In other words, rain, instead of running off, enters the holes, and so penetrates the earth, thus being held for a longer time. Air also finds its way below the surface; so that the homes of the little creatures constitute storehouses for moisture.