LITMIR.BIZ

      SCALARIA.

      I have already mentioned some thread-spinners among the Mollusca; there are others which have ​the power of forming threads of silky substance much stronger and more durable than those of our pond snails. The Common Mussel (Mytilus edulis) is one of these marine silk-worms; and we have a good many others. The bundle of threads, familiar to many of my readers as the beard of the shell-fish, is the substance in question, termed by naturalists byssus, a Greek word originally signifying silk; and the use to which it is applied by the animal itself is that of a cable to moor itself to the solid and immovable rock, that it may not be washed away by the violence of the waves. The mode in which the threads are formed, and the organ by which they are secreted, are thus described by Professor Rymer Jones:—

"The foot in the Mussel is of small dimensions, being useless as an instrument of progression. By its inferior aspect it gives attachment to the horny threads of the byssus, which are individually about half an inch in length, or as long as the foot itself, by which, in fact, they are formed, in a manner quite peculiar to certain families of Conchifera; no other animals presenting a secreting apparatus at all analogous, either in structure or office, to that with which these creatures are provided. The manner in which the manufacture of the byssus is accomplished is as follows: A deep groove runs along the under surface of the foot, at the bottom of which thin horny filaments are formed by an exudation of a peculiar substance, that soon hardens and assumes the requisite tenacity and firmness. While still soft, the Mussel, by means of its foot, applies the extremity of the filament, which is dilated into a kind of little sucker, to the foreign substance whereunto it wishes to adhere, ​and fastens it securely. Having accomplished this the foot is retracted; and the thread, of course, being drawn out of the furrow where it was secreted, is added to the bundle of byssus previously existing, all of which owed its origin to a similar process."^[16]

      Whoever has attempted to wrench up a Mussel from one of those shallow rock-pools, in which they lie as closely packed as paving stones, will have had proof of the great strength of these threads, no small violence being required to detach one. But there is an example on record, where the strength of the threads has been turned to such account as to give this Mollusk a second claim to be included in the list of such species as are beneficial to man.—"At the town of Bideford, in Devonshire, there is a long bridge of twenty-four arches across the Torridge river, near its junction with the Taw. At this bridge the tide flows so rapidly that it cannot be kept in repair by mortar. The Corporation, therefore, keep boats in employ to bring mussels to it, and the interstices of the bridge are filled by hand with these mussels. It is supported from being driven away by the tide entirely by the strong threads these mussels fix to the stonework; and by an act, or grant, it is a crime liable to transportation for any person to remove these mussels, unless in the presence and by the consent of the corporative trustees."

      There are bivalve shells allied to the mussel, called Pinna, usually of very large size, but of thin and delicate structure. The threads spun by these are long, fine, glossy, and produced in great abundance; they are capable of being twisted like silk, ​and the inhabitants of Sicily weave them into a sort of cloth remarkable for its softness and warmth, but which refuses to take any dye. In the British Museum, together with some very fine specimens of the shells of this Mollusk, there is a pair of gloves made of its byssus; but articles made of this material are very costly, and cannot be considered in any other light than that of curiosities. Pope Benedict XV, in 1754, had a pair of stockings presented to him which were woven from the silk of the Pinna. These were the subject of general admiration, from the extreme delicacy of their texture—well shown by the minuteness of the box in which they were enclosed.

      The mention of the ship-worm naturally presents to the mind another tribe of boring Mollusca—those which perforate hardened clay, and even stone. These, belonging to various genera, are sufficiently common on our own coasts. Different species of Pholas excavate their burrows, which resemble the holes bored by augers or large gimlets in wood, clay, and sandstone; the Venerupis in shale and similar friable rocks, the Lithodomi and Saxicavæ in the limestone, and the Gastrochæna in limestone, fluor, and granite. A curious example of the boring powers of one of these species, the Modiola lithophaga occurs at Pozzuolo, in the Bay of Naples, where a colony of these Mollusks had settled themselves in the pillars of the temple of Jupiter Serapis during the period of its submersion. At the height of ten feet above the base of the three standing pillars which remain, and in a position exactly corresponding in all, is a zone of six feet in height, where the marble has been scooped into cells by these Mollusca. The holes are to the ​depth of four inches; and it is observed that the nodules of quartz and feldspar, which sometimes occur in the hard limestone of the pillars, are untouched.

Many theories have been invented to account for the singular power exercised by these animals, such as the following; that the animals entered the rock while it was in a soft and plastic state, and that it afterwards hardened around them—that the animal poured out some peculiar fluid which had the chemical property of dissolving the rock—that the latter was ground away by the roughnesses on the shells as they revolved, as if by the action of a rasp or file—that the minute particles of the stone were one by one separated and driven off by the force of currents of water, produced by vibrating cilia: but all these theories appear to be set aside by the discovery of Mr. Albany Hancock, one of the highest living authorities on the subject. This gentleman finds that the excavating instrument is the anterior portion of the animal, either the foot and the edges of the mantle, or the edges of the mantle solely. These organs are fitted for the office they are to perform, not only by their position and figure, and their pliability and muscular structure—made more than commonly muscular for the duty—but also by being armed with a rough

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