The vast number of shells and the varieties of each kind can hardly be realized by those who have not examined a well-equipped collection. Over four thousand species of the mussel are known, and hundreds of species of almost every shell exist in various streams and seas. The shells range from minute specimens hardly visible to giants weighing several hundred pounds, one of the latter being the huge clam, Tridacna (Fig. 88), found in the equatorial Pacific. There are several species, and in the largest each valve weighs about two hundred and fifty pounds. The animal itself weighs thirty pounds, and affords a meal to forty or fifty men. The shell, by means of its foot, buries itself in the soft rock of the regions in which it lives. With its valves partly open it resembles a huge sea anemone; but it closes them at the slightest alarm. Large fishes, and even natives, it is said, have been trapped by this giant, whose jawlike valves, with three huge teeth, grip the fin of a fish or the foot of an unfortunate wader with a vicelike grasp. The byssus or anchor of this huge shell is so thick and tenacious that it is severed only with great difficulty and labor. The shells are valuable as ornaments, large numbers being sent to various countries for this purpose. The giant never moves, and in this respect is a sharp contrast to the little donax, so common on our various shores and in France, which leaps along the muddy flats by convulsive movements of its fleshy foot. [Illustration: FIG. 89.--Razor clam.] The common razor clam, of which sixty or more species are known, by means of its foot (Fig. 89) digs a deep burrow which is filled with water even at low tide. The shell is often found at the entrance, but at the slightest alarm it dashes deep down into its den, to be caught only by persistent digging. The odd shapes assumed by many bivalves is well illustrated in the hammer oyster (Fig. 90) and the pholas. The latter illustrates the power of the most insignificant animals, as by means of its foot this little shell burrows into the hardest granite. It is invariably found there and imprisoned; for when it reaches the interior of a stone, it grows and enlarges, leaving but a small opening for the siphons. It is supposed by some that the pholas possesses some secretions by which it dissolves the stone, and by others that it wears away the rock by using its shell as a file. In any event the shell is known to contain aragonite, a very hard substance. In the pillar of the temple of Serapis, Italy, the holes made by this shell are seen. [Illustration: FIG. 90.--Hammer oyster.] Perhaps the most remarkable feature of the pholas is its power as a light giver. It emits a delicate blue light, dead or alive. One placed in a glass of milk has been used as a lamp, illuminating the faces near it. Another placed in honey retained its phosphorescence for over a year. The little pholas is found all over the world, more than eighty different species being known.