Bivalves move about and change from place to place by means of an extensible fleshy organ called, from some of its functions, a foot; in fact, it has less resemblance to a foot than to a large tongue. It is a muscular mass, capable of being pushed out from between the mantle and the valves, and varies much in form; it is in turn a hatchet, a ventilator, a pole, an awl, a finger, and a sort of whip. This foot is simple, forked, or fringed. In some species the tissues are spongy, and capable of receiving considerable quantities of water. When the organ swells, it is elongated and stiff; on the other hand, by suddenly expelling all the water, it gets small and pliable, and can now return to its shell. This organ is represented in Fig. 128 (_Donax trunculus_, Linn.), in which it is singularly developed. This bivalve is found on the sea shore in shallow water; it buries itself almost perpendicularly in the sands. They are so abundant on the French side of the Channel and on the shores of the Mediterranean, that they form a considerable portion of the people's food. These bivalves have the singular power of leaping to a considerable height and then throwing themselves to a distance of ten or twelve inches--a spectacle which may be witnessed any day at low water. When abandoned by the retreating tide, they try to regain the sea. If seized by the hand, in order to drag them out of the sand, aided by their compressed, branched, and angular feet, they give to their shell the sudden and energetic movement under which the bounding action takes place. [Illustration: Fig. 128. Donax trunculus (Linnæus).] The shell of the Donax is slightly triangular and compressed; its length exceeds its height; it is regular, univalve, unequally lateral, and its hinge bears three or four teeth on each valve. The action of these feet is very simple, and is compared by Réaumur to that of a man placed on his belly, who, stretching out one hand, seizes upon some fixed object, and draws himself towards it. There is just this difference, that the movement of the member in the mollusc is altogether contractile. Authors have described more than 30,000 species of molluscs, so that our space only permits us to describe a few families, or rather types of families. The arrangement of bivalves now most generally adopted in England is that of Woodward, as developed in the last edition of his manual of the mollusca; it is greatly based on that of Lamarck. We have adopted his arrangement altered from a descending to an ascending scale of organisation. The Conchifera are divisible into two sections, Siphonida, from the animals having respiratory siphons, and Asiphonida, destitute of them.