Up to the present time the faculty of pro ducing a byssus is not known to belong to any other class of animals, and it is limited to a few only of the Conchiferous mollusks. Among the Dimyaria the genus Byssomya may be quoted as an example, also the members of the family of the Mytilacea ; and, if the horny plates of certain Archm be likened to the byssus, it would also be necessary to include this genus in the group of byssiferous Dimyaria. In the Monomyaria provided with a foot, the whole of the genera are byssiferous, with the exception of those which attach themselves im mediately by their shell.
The byssus (b, fig. 353) is a bundle of horny or silky filaments, of different degrees of fine ness and of different thicknesses, and flexible in various measures, by means of which the animal is, as it were, anchored to any solid body sunk in the sea. The filaments, for the most part distinct from one another, are, how ever, occasionally connected into a single mass of a subcylindrical form, and terminated by a broad expansion, which serves as the point of attachment. This disposition is to be ob served in the Aviculce, and leads to the belief that the horny mass of certain Aram is a mere modification of the byssus. In those species of which a byssus is formed of separate filaments, these are all seen to be detached from a com mon pedicle (c,fig. 353), situated at the infe rior base of the foot (d, fig. 353). If the byssus be examined before any of the filaments are torn, it is easy to perceive that these are attached to submarine bodies by means of a small disc-like expansion of their extremities, of various extent according to the genus and species (a, a, a,fig. 354). Attentive examina tion of these filaments shews that they are of equal thickness through their entire length, and that they have nothing of the structure of the hair of the higher animals.
If the byssus and foot of a byssiferous mol lusk be placed under a powerful lens, the last filaments of the byssus are first seen to be nearest to the base of the foot; and if the infe rior edgl of the foot be inspected, a fissure will be found running completely along it, at the bottom of which a brownish and semi-corneous filament is often to be perceived ; this is neither more nor less than a filament of the byssus prepared to be detached by the animal, in order to which the animal stretches forth its foot until it encounters the object upon which the other fibres of the byssus are fixed ; to this it applies the point of the foot: which then se cretes a small quantity of glutinous matter, continuous with the silky filament lying along the bottom of the furrow of which we have spoken. When the pasty matter has acquired sufficient consistency, and is firmly fixed to the stone or other body at the bottom, the animal retracts its foot, and in doing so detaches the new fibre to the base of the pedicle. The mode in which the filaments of the byssus are formed, is consequently entirely different from that in which hair or the horns of the higher animals are evolved, and it is easily under stood when the intimate structure of the foot of the byssiferous mollusks is known, when we are aware that this organ consists in its centre of a pretty considerable fasciculus of parallel and longitudinal fibres. By a faculty peculiar
to the class of animals that now engages our attention, the fibres situated at the bottom of the groove of the foot become horny, and are detached in succession in the form of threads as tliey become consolidated. Certain genera are celebrated for the abundance and fineness of the byssus ; that of the Pinnw, among others, which was even known to the ancients, may be spun into threads like silk or wool, and may be used to manufacture tissues of an unchangeable colour, and of great strength and durability.
With reference to form, the foot presents a variety of interesting modifications. Some times it is short and truncated, as in the genus Pholas; sometimes more elongated, but still truncated at the summit, as in certain Razor-shells ( Solen), (a, fig. 355) ; in which the edges of the truncation are regu larly toothed. A few of the acephalous mollusks have the foot cylindrical (a, fig. 356), as the So lenes ; when it presents this form, the organ is generally terminated by a kind of glutinous point, or disc, which enables the animal to fix itself at different heights in the deep cylindrical hole it digs for itself in the sand. The foot, which is shaped like a tongue, is named linguiform, as in the Solen strigilatus; it is elavifbrm when it is thicker at its extremity than at its base : it is found of this shape in certain other Solens. The foot again is vermiform when it is very slender and much elongated, as in the Loripes and Lima. When it is thus formed, it appears to us to be incapable of subserving motion. In a considerable number of species the foot is conical, as in the Cockle, (a, fig. 357) ; and in this case it is generally folded into two nearly equal portions, so that by its means the animal can leap pretty actively. It is secu riform when its free edge is arched like the cutting face of an axe, as in Petunculus,'(a, fig. 358). When it presents this form its edge is generally divided into two lips, which, being separated, present with some degree of ac curacy, although much contracted, the sem blance of the locomotive plane of certain Gas teropoda. When this structure occurs, the foot is said to be bifid, as in Nucula, Trigonia. It is said to be flattened when it is thin and laterally depressed, as in Tellina and Donax ; to conclude, it is designated as bent when it consists of two portions connected at an angle with one another (b, fig. 359), of which the genera Cardium, Nucula, and Trigonia present examples. Various other modifications, of less importance than those we have particularized, also occur; these can be aptly- enough alluded to in the anatomical description.