The organ denominated foot in the acepha lous mollusks is a part which presents very different forms, and is destined to locomotion. This part is particularly well developed among the Dimyaria, and we shall pass in rapid re view its most general features.
The foot (b,fig. 346) is usually situated at the anterior and middle part of the abdominal mass, and is directed forwards. It is so placed as to hide the mouth in a deep sinus between its base and the anterior adductor muscle. In those conchiferous mollusks in which the lobes of the mantle are united through a great por tion of their circumference, the foot is com monly very small and meiely rudimentary ; it then forms a kind of little nipple projecting from about the middle of the abdominal mass, a form which is very distinctly seen in the Mya, Saxicava, &c. In others of these mol lusks the foot, more anteriorly situated, is ex tremely short, broadly truncated, and similar to a cupping-glass; this configuration is observed in the Pholadia. In proportion as the foot be comes more free, the lobes of the mantle are distinct from one another, the foot becomes flattened and elongated in the form of the human tongue, and is subservient to motion by digging a hole or furrow in the sand into which the animal sinks. This form of the locomotory organ is met with more especially in the Tellina, the Donata, and a very great number of other genera, the shells of which are more or less flattened. Lamarck had attached some consequence to the shape of the organ of locomotion, and Goldfuss has proposed a clas sification based upon the modifications pre sented by this organ ; but the groups establish ed in accordance with such considerations are in reality of no importance; the several forms proper to the organ pass too insensibly one into another to make it possible to say where one terminates and another begins ; the boundary between one family and another, with a few rare exceptions, is altogether indefinite. In the present day, consequently, naturalists no longer admit into their methods of arrangement the groups established by Lamarck under the names of Tenuipeda, Crassipeda, &c.
The foot exists developed in a greater or less degree in the whole of the Dimyaria. If in some species it is found merely rudimentary, it is yet never altogether wanting in any immber of this first division of the Conchifera. The organ is also met with in a very considerable number, but by no means in the whole of the Monomyaria, and the presence or absence of the foot might be taken as the basis of a division of this great family into two series, in the one of which the foot was rudimentary but present, whilst in the other it vvas no longer to be found.
Whatever the form of the locornotory organ, and whatever the degree of its development, it is always organized in the same manner. It is essentially composed of several planes of mus cular fibres (1, 2, 3,fig. 347), which by their
various courses and interlacements enable it to perform a great variety of difirent motions, either in part or as a whole. When the foot is short or vermiform, its mass is entirely muscu lar from the apex to the base. It is at the base that the fleshy fibres separate into two fasciculi (4, 5, fig. 347), which, after having. circum scribed the visceral mass, proceed backwards, where they are attached to each valve of the shell near the implantation of the posterior ad ductor muscle in the Dimyaria; and towards the superior part of the valves, and occasion ally in the interior of the hook, or incurved part of the shell in the Monomyaria.
In the Conchifera denominated Lamellipeds and Crassipeds by, Lamarck, in a word, in the whole of the Conchiferous mollusks in which the foot constitutes a principal part of the body, this orga.n presents remarkable differences in its composition and its rela tions with the internal organs. It is then formed of two lateral planes of fibres, uniting and blending together near the free edge. These two planes, more or less separate ac cording to the general form of the animal, have between them an internal space, within which is included a considerable portion of the visceral mass. In the generality of conchife rous mollusks furnished with a large foot, it is here that a portion of the liver is situated, the greater part of the intestinal canal, and a notable portion of the ovary. These organs are bound down in the place they occupy, and the, parietes of the foot are preserved in immediate communication by means of a great number of small muscles, sometimes straight, sometimes oblique, and variously interlaced, to which Poli has given the name of funicular muscles (j,j, fig. 347). They are particularly conspi cuous in the cylindrical foot of the Solens, in the flattened foot of the Tellime, and of the Uniones, and they have a remarkable arrange ment in that of the Cardiw. They appear to be wanting in the foot of those Conchiferous mollusks that attach themselves by means of a byssus. In them the foot is reduced to the functions of spinning (de filer) the threads of the byssus, and it is not therefore surprising that its organization should be found to be peculiar. Reduced to a purely rudimentary state, the foot in the Monomyaria (b, fig. 3.48) appears rather as an appendage to the mass of the viscera than as their defensive envelope. The muscular fasciculi that terminate it pos teriorly are small; they pass through the vis ceral mass to be attached either to the superior part of the central muscle, or within the in terior of the hooks or beaks of the shell. Almost the whole of the Monomyaria furnished with a foot, have a byssus also ; to this rule there are indeed a small number of exceptions, among others the Limee.