Many of the most beautiful of the marine worms are cell builders (Fig. 77). In some the worms secrete a tube of carbonate of lime. In others the den is made of bits of sand. I found on the Florida Reef many remarkable examples of the latter. The nest or tube was built among the seaweed, several inches above the bottom, and would naturally be a conspicuous object; but here the intelligence of the little creature is seen, for it covers the outside of the column with the plates of a lime-secreting seaweed, which look like shingles, and mounts upon the upper portion of the column a green bit of seaweed. This is glued to the tube and so arranged that it falls over the entrance and closes it, thus serving the purpose of a door and making the tube mimic a bit of sea grass. The worm lifts the grass door when it comes out. [Illustration: FIG. 78.--A group of tube-secreting worms (_Serpulæ_).] One of the most familiar forms is Serpula (Fig. 78), whose tubes wind in and out in every direction. No garden of pansies gives a greater variety of tints than did a mass of these radiant creatures that I found on a floating spar in the Pacific at Avalon Bay. But touch these "flowers" or jar them and they disappear like magic, leaving a hole closed by a little door, which is formed by a part of the worm that thus defies all intruders. XI. THE TWO-VALVED SHELLS [Illustration: FIG. 79.--The oyster: _A_, muscle; _B_, mantle; _C_, gills; _D_, labial palpi; _E_, hinge; _F_, mouth; _G_, liver and stomach; _H_, heart.] The beautiful objects which we know as shells, and which form ornaments in many a home far distant from the sea, are the coverings of a group of animals called mollusks. They are found in all seas, many upon land, and in fresh-water streams, and are among the most attractive of all natural objects, so much so that many persons devote their entire lives to their collection, and many others much time to the study of their habits. It is rare to find a new shell, so well have these shell hunters searched the waters of the world. Such collections, especially if complete, are very valuable, and many of the great museums have paid thousands of dollars for them. [Illustration: FIG. 80.--Anatomy of snail: _a_, mouth; _b_, foot; _c_, anus; _d_, lung; _e_, stomach; _f_, intestine; _g_, liver; _h_, heart; _i_, aorta; _j_, gastric artery; _k_, foot artery; _o_, lung and heart artery.]