Solen, or Razor-Shell.
a. Foot.
In several of the sedentary genera the rudimentary foot, though incapable of locomotion, makes itself useful by spinning a bundle of silken threads, called byssus, or beard, which serve to anchor the animal to any solid submarine object as firmly as a ship in harbour. Generally the connection is permanent, but some species, among others the edible mussel, are able to detach the filaments from the glandular pedicle situated at the inferior base of the foot which originally secreted them, and then to seek another point of attachment.
If the byssus be examined under a powerful lens, before any of the filaments are torn, it is easy to perceive that these are fixed to submarine bodies by means of a small disc-like expansion of their extremities of various extent, according to the genus and species. Certain genera are celebrated for the abundance and fineness of their byssus; that of the Pinnæ, or Wing-Shells, among others, which are very common in some parts of the Mediterranean, and attain a considerable size, is so long and firm that in Naples it is sometimes manufactured into gloves and other articles of dress, though more as an object of curiosity than for use.
Thus we find in the same class of animals the same organ most variously modified in form and structure; now serving as a foot, now as a spade, or as a rasp, or as a spinning machine, and, throughout all these modifications, admirably adapted in every case to the mode of life of its possessor.
Pinna.
c. Pedicle from which the filaments are detached.
d. Inferior base of the foot.
The whole construction, and generally the extremely restricted locomotion, of the bivalves tells us at once that they are unable to attack their prey, but must be satisfied with the food which the sea-currents bring to the door of their shells, or within the vortex of their branchial siphons. But they have as little reason to complain as the equally slow or sessile polyps, bryozoa, and ascidians, for the waters of the ocean harbour such incalculable multitudes of microscopic animals and plants that their moderate appetite never remains long unsatisfied. The same streams which aërate their blood also convey to their mouth all the food which they require.
Deprived of more active weapons, most bivalves rely upon their shells as their best means of defence, and to answer this purpose, their stony covering must naturally increase in solidity the more its owner is exposed to injury. The pholades, lithodomes, and teredines, which scoop out their dwellings in stone or wood, and thus enjoy the protection of a retrenched camp, can do with a thin and brittle or even with a mere rudimentary shell. The solens, which at the least alarm bury themselves deeper and deeper in the sand, likewise require no closely-fitting valves; but the oysters or mussels, which have no external fortress to retire to, and are unable to move from the spot, would be badly off indeed if they could not entirely conceal themselves within their thick shells, and keep them closed by strong muscular contraction.
Bernardin de St. Pierre, in his "Studies of Nature," points out another admirable provision for the safety of molluscs. Thus, those which crawl and travel, and can consequently choose their own asylums, are in general those of the richest colours. Such, among the Gasteropods, are the gaudily-tinted Nerites, and the polished marbled Cowries, the Olives, richly ornamented with three or four colours, and the Harps, which have tints as rich as the most beautiful tulips; while among the bivalves the vivacious Pectens, coloured scarlet and orange, and a host of other travelling shells, are impressed with the most lively colours. But those which do not swim, as the Oysters, which are adherent always to the same rocks, or those which are perpetually at anchor, as the Pinnas and Mussels, or those which repose on the bosom of Madrepores, such as the Arcs, or those which are entirely buried in the calcareous rocks, as the Lithodomi, or those which immovably, by reason of their weight, pave the surface of the reefs, as the Tridacna, are of the colour of the bottoms or floors which they respectively inhabit, in order, no doubt, that they shall be less perceived by their enemies.
But even so the best guarded of the bivalves fall a prey to innumerable enemies, and when we see the strand covered for miles and miles with their débris, we may rest assured that but few of the quondam inmates of these fragmentary shells have died a natural death. Annelides and Sea-snails, crustaceans and star-fishes, strand birds and even quadrupeds, all fatten upon their delicate flesh, and man devours incalculable numbers.