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.
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.
Fig. 91.—Teredo, a boring shell.
The teredo or shipworm (Fig. 91) is called a worm because it secretes a limy shell, but it is really a bivalve shell open at both ends, a shell which with one exception causes more destruction than all other marine animals combined. Instinctively it bores into wood, forming an irregular tunnel and lining it with a delicate coating of carbonate of lime. Some years ago I visited on the outer Florida Reef, an old wreck which was newly buried in the sand and partly exposed at low tide. The timbers of the vessel looked strong and able to stand many a storm, yet with a blow of my hand I broke through the planking. The interior was completely honeycombed by the teredo, so that it was a maze of tubes. At this place the life of a pile of yellow pine was a year and a half; in other words, after being exposed to the teredo for this length of time, it was useless. On the Pacific, at Avalon Bay, the piles last about two years, being rapidly destroyed, even though soaked in various poisonous fluids and coated with tar. Many thousands of dollars have been expended in experiments with devices to outwit the teredo, but without avail, and they are the greatest menace to navigation and piers to-day, making their way into hulls, despite the copper sheathing. In the mud banks of the waters of Sumatra, teredos are found which attain a length of six feet, with tubes four inches in diameter.