The pectens are common forms famous for the beautiful gemlike eyes seen along the edge of their mantles. I once kept a number of these shells in an aquarium, and they were a source of much amusement, from their habit of dancing (Fig. 86). Generally they lay in the sand in the bottom of the tank with their valves open an inch or more, their bright eyes gleaming. Without any warning, one would open and close its valves with great rapidity, which would cause the shell to take convulsive and bounding hops. Then another shell would follow, and soon all the pectens were leaping up and down in a most extraordinary dance. The pecten changes its position or travels, not by pushing itself along, but by a sudden and spasmodic hop, clearing a foot or more. [Illustration: FIG. 87.--Mussel climbing: _B_, cables; _F_, foot.] The locomotion of shells itself is a fascinating subject. An interesting instance is observed in the common mussel. This shell has a remarkable foot, a pointed, fleshy organ which can be protruded. With this organ the mussel bores holes in the sand, jerks itself along, or clears the surface with a bound. But its most remarkable service is in aiding the mussel to climb. In the foot, near its base, is a gland which secretes a peculiar substance, which when exposed to the water hardens and resembles silk. The resemblance is so perfect that the "silk" has been woven into various articles, and an attempt was made in France to raise mussels for this purpose. When the animal desires to climb, it reaches out its foot as high as it can (Fig. 87), and presses it upon the pile or rock, whereupon a delicate cord, one of the cables of its byssus, is seen. Again the foot is extended, again a cable is attached, the entire operation calling to mind the action of a spider. Each step raises the mussel a little higher, and as it moves on, the cables that would hold it back are broken off, and the mussel at length reaches the position it desires. The fresh-water mussels found in the Ohio and other rivers and streams are pearl producers. Very valuable gems have been taken from them in various states, and the fresh-water pearl fishery of the United States is of considerable importance. A fresh-water pearl found in New Jersey was valued at $2000, and one taken from a stream in Scotland brought $50,000. [Illustration: FIG. 88.--Giant clam.]