When leaves, petioles, sticks, etc., cannot be obtained for the mouths of their burrows, heaps of stones, smooth, rounded pebbles, are utilized for protection. When the stones are removed and the surface of the ground is cleared for some inches round the burrow, the worms may be seen with their tails fixed in their burrows dragging the stones inward by the aid of their mouths, stones weighing as much as two ounces often being found in the little heaps, which goes to show how strong these apparently weak creatures are. Work of this kind is usually performed during the night, although objects have been occasionally known to be drawn into the burrows during the day. What advantage worms derive from plugging up the mouths of their burrows, or from piling stones over them, cannot be satisfactorily answered. They do not act in this manner when they eject much earth from their burrows, for then their castings serve to cover the mouth. Perhaps the plugs serve to protect them from the attacks of scolopenders, their most inveterate enemies, or to enable them to remain with safety with their heads close to the mouths of their burrows, which they like so well to do, but which, unless protected, costs many a fellow its life. Besides, may not the plugs check the free ingress of the lowest stratum of air, when chilled by radiation at night, from the surrounding ground and herbage? The last view of the matter seems especially well taken, because worms kept in pots where there is fire, having no cold air with which to contend, plug up their burrows in a slovenly manner, and because they often coat the upper part of their burrows with leaves, apparently to prevent their bodies from coming into contact with the cold, damp earth. But the plugging-up process may undoubtedly serve for all these purposes. Whatever the motive may be, it seems that worms much dislike leaving the mouths of their burrows open, yet, nevertheless, they will reopen them at night, whether or not they are able afterwards to close them.