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 layer of numerous crystalline particles of various sizes and shapes, chiefly five- and six-sided, and all having one or more elevated points near the centre. These crystals are imbedded in the surface of the boring foot and thickened edges of the mantle; and, consisting, probably, of silex or flint, either pure or in combination with some animal matter, they form a sort of file—superior, however, to any of our workmen's files in this, that the surface keeps itself always in a proper state of roughness for trituration. This is done by an organic law, which causes the crystals to be constantly shed, and as constantly renewed.[17]
LIMPET.
All the borers above alluded to are Bivalves, and I know of no other Mollusk which can properly be classed with them. A common Gasteropod, however, the familiar Limpet (Patella vulgata), excavates the rock on which it lives to the extent of making a depression, more or less deep, exactly corresponding to the shape and size of the margin of its shell. When one removes a Limpet from its firm adhesion and finds a hollow beneath it, evidently made to contain its body, one is ready to conclude that the animal is a permanent tenant of the spot, never moving from it; and when we learn that the food of the Limpet consists of sea-weeds, we wonder how it is possible that a stationary animal can find vegetable food. But the truth is, I believe, that the Limpet wanders away from its hollow during the night, returning to it as a home by an infallible instinct on the approach of morning. The mode in which the excavation is performed is the same as that just mentioned in the case of the borers, the whole under surface of the foot being furnished with sharp crystals of flint imbedded in its substance.
In general the stony shells of the Mollusca afford them, a sufficient protection, but a few species construct for themselves nests. A native example of this instinct is described in interesting terms by the Rev. D. Landsborough, who obtained it in Lamlash Bay:—
"The most interesting, though not the rarest, thing we got was Lima hians. I had before this some specimens of this pretty bivalve, and I had admired the beauty and elegance of the shell; but hitherto I had been unacquainted with the life and manners of its inhabitant. Mr. and Miss Alder had got it in the same kind of coral at Rothesay, so that when Miss Alder got a cluster of the coral cohering in a mass, she said, 'O, here is the Lima's nest!' and breaking it up, the Lima was found snug in the middle of it. The coral nest is curiously constructed, and remarkably well fitted to be a safe residence for this beautiful animaL The fragile shell does not nearly cover the Mollusk, the most delicate part of it, a beautiful orange fringe-work, being altogether outside of the shell. Had it no extra protection, the half-exposed animal would be a tempting mouthful—quite a bonne-bouche to some prowling haddock or whiting; but He who tempers the wind to the shorn lamb, teaches this little creature, which He has so elegantly formed, curious arts of self-preservation. It is not contented with hiding itself among the loose coral, for the first rude wave might lay it naked and bare. It becomes a marine-mason, and builds a house or nest. It chooses to dwell in a coral grotto; but in constructing this grotto it shows that it is not only a mason, but a rope-spinner, and a tapestry-weaver, and a plasterer. Were it merely a mason, it would be no easy matter to cause the polymorphous coral to cohere. Cordage, then, is necessary to bind together the angular fragments of the coral, and this cordage it spins; but it spins it as one of the secrets of the deep. Somehow or other, though it has no hand, it contrives to intertwine this yarn which it has formed, among the numerous bits of coral, so as firmly to bind a handful of it together. Externally, this habitation is rough, and therefore better fitted to elude or to ward off enemies. But though rough externally, within all is smooth and lubricous, for the fine yarn is woven into a lining of tapestry, and the interstices are filled up with a fine slime, so that it is smooth as plaster-work …
"When the Lima is taken out of its nest, and put
