The threads issue from the shell at that part where it naturally opens, and in affixing themselves to any substance, form numerous minute cables, by which the fish steadies itself in the water. Each animal is provided with an organ, which it is difficult to designate by any name, since it performs the office of so many members, and is the only indicator of the existence of vital powers in the creature. It is by turns a tongue, an arm, and sometimes a leg. Its shape resembles that of a tongue, and is, therefore, most frequently called by that name. Whenever the fish requires to change its place, this member serves to drag its body forward, together with its cumbrous habitation: in performing a journey, the extremity of this organ, which may then be styled a leg, is fixed to some solid body, and being then contracted in length, the whole fish is necessarily drawn towards the spot where it intends to station itself; and by a repetition of these movements, the animal arrives at its destination. It is not often that the organ is put to this use, as the pinna is but little addicted to locomotion: some naturalists indeed affirm that it is always stable. The purpose to which the tongue is most frequently applied, is that of spinning the threads. Although this body is flat, and in form similar to a tongue through the greater part of its length, it becomes cylindrical about the base or root, where it is much smaller than in any other part: at this lower end are several ligatures of a muscular nature, which keep the tongue firmly fixed against the middle of the shell; four of these cords are very apparent, and serve to move the tongue in any direction according to the wants of the fish. Through the entire length of this member there runs a slit, which pierces so deeply into its surface, as almost to divide it into two longitudinal sections; this performs the office of a canal for the liquor of which the threads are formed, and serves to mould them into their proper form: the canal appears externally like a small crack, being almost covered by the flesh from either side, but internally it is much wider, and surrounded by circular fibres. The channel thus formed extends regularly from the tip to the base of the tongue, where it partakes of the form of the member and becomes cylindric, producing there a tube or pipe in which the canal terminates. The viscid substance is moulded in this tube into the shape of a cord, similar to the threads produced from it, though much thicker, and from which all the minute fibres issue and disperse. The internal surface of the tube, wherein the large cord is formed, is furnished with glands for the secretion of the peculiar substance employed in its production, and which is always in great abundance in this animal as well as in muscles.