This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1299). Diverse are the uses to which the foot may be turned. It is generally used for burrowing in the sand or soft mud; and, by its constant and worm-like action, those species in which it is largely developed can bury themselves with facility, and make their way beneath the sand with a dexterity not a little remarkable. Perhaps the most efficient burrowers met with upon our own shores are the Razor-shells (Solenidae), in which family the fleshy foot attains to enormous proportions; and the rapidity of their movements beneath the soil will be best appreciated by those who may have watched the manner in which the fishermen effect their capture.
(1300). The Solen excavates for itself a very deep hole in the sand, boring its way by means of its foot to a depth of some feet, and remains concealed in this retreat, usually occupying a position within a few inches from the surface. The fisherman, armed with a slender iron rod furnished with a barbed head, resembling a harpoon, treads carefully backwards over the beach left bare by the retreating tide, and finds the holes in which Solen lodges by watching the little jet of water thrown out by the animal when, being alarmed by the shaking of the sand, it contracts its body. Guided by the orifice through which the water is thrown, he plunges his rod into the sand, and generally succeeds in piercing the animal with the barbed extremity, and dragging it from its concealment; but should he fail in his first attempt, he well knows that to try again would be unavailing, for the animal instantly works its way down to such a distance as to render pursuit hopeless.
(1301). But, however efficient as a means of burrowing the foot may be, it can be turned to other purposes. The Pholades, for example, by some means, either of a mechanical or chemical nature, not as yet precisely determined, excavate the solid rocks, and form therein chambers in which they pass their lives. In such genera, the foot, which would be useless as a boring instrument, by being simply transformed into a broad and flat disk becomes a powerful sucker, whereby the Pholas fixes itself to the walls of its apartment in any convenient situation.
(1302). In many of the Cockle-tribe we find the foot converted into an instrument of locomotion of a very singular description, enabling the cardiaceous Conchifera to leap by bounds we should scarcely expect animals so unwieldy to be capable of executing. For this purpose the end of the foot is bent, and placed firmly against the plane of support, in the position represented in fig. 252; when thus fixed, a sudden spring-like action of the muscles of the foot throws the cockle into the air, and, by a repetition of these exertions, the creature can skip about with surprising agility.
(1303). But the most extraordinary office assigned to the foot in the class under consideration is the manufacture of horny threads, whereby, as by so many anchors, the Mollusca thus provided fix themselves securely to foreign bodies, and that so firmly, that extraordinary violence is requisite to wrench such animals from the place where they have fixed their cables. The marine Mussel is a well-known example of a byssiferous Mollusk; and from this species, therefore, we shall draw our description of the organs by which the tough filaments referred to are secreted.
(1304). The foot in the Mussel is of small dimensions, being useless as an instrument of progression. By its inferior aspect it gives attachment to the horny threads of the byssus, which are individually about half an inch in length, or as long as the foot itself, by which, in fact, they are formed in a manner quite peculiar to certain families of Conchifera - no other animals presenting a secreting apparatus at all analogous, either in structure or office, to that with which these creatures are provided. The manner in which the manufacture of the byssus is accomplished is as follows: - A deep groove runs along the under surface of the foot, at the bottom of which thin horny filaments are formed by an exudation of a peculiar substance, that soon hardens and assumes the requisite tenacity and firmness. While still soft, the Mussel, by means of its foot, applies the extremity of the filament, which is dilated into a kind of little sucker, to the foreign substance whereunto it wishes to adhere, and fastens it securely.
Having accomplished this, the foot is retracted; and the thread, of course, being drawn out of the furrow where it was secreted, is added to the bundle of byssus previously existing, all of which owed its origin to a similar process.
(1305). Sometimes, instead of the numerous thin filaments met with in the Mussel, the byssus consists of a single, thick, horny stem; while in other cases, as, for example, in Pinna, the threads are so numerous, soft, and delicate, that they are not unfrequently spun like silk, and manufactured into gloves and other small articles of dress not unfrequently met with in the cabinets of conchologists.
(1306). Taking a more general view of the Conchiferous Mollusca than we have hitherto done, we shall now proceed to consider the mechanism for opening and closing the valves of the shell in which they reside, - an operation effected in a very simple and elegant manner.
(1307). The shells are connected posteriorly by means of a hinge, differently constructed in different species. In the Oyster we have an instance of the most simple kind of junction. In these Mollusca, a mass of elastic ligament, composed of perpendicular and parallel fibres, is interposed between the posterior edges of the shell, and so disposed that, by closing the shell, the ligamentous mass is forcibly compressed, while at the same time its resiliency is such that, immediately the compressing power is withdrawn, it expands, and thus forms a simple spring calculated to keep the valves apart and cause their separation to a greater or less extent.
(1308). The antagonist to this elastic force is the adductor muscle (fig. 247, c), a fleshy mass of very great strength, the fibres of which pass directly from one valve to the opposite. The adductor muscle, although in this case single, consists of two portions of different texture (fig. 248, I, m); so that it would appear to be formed by two muscles closely approximated so as to compose a single powerful mass adapted to keep the valves in contact, with a force proportioned to its massive size. All those species having a single muscular mass, such as the Oyster and Pecten, have been grouped together by conchologists under the general name Monomyaeia, while another and more numerous division, Dimyaria, is characterized by having two adductor muscles, distinct and widely removed from each other. The Mussel-tribe and many others are examples of this arrangement, which is represented in subsequent figures.