Further, the inner surface of each valve is marked by the impressions or scars of other muscles, the number and position of which vary considerably in different species. They include the adductor muscle or muscles (one or two in number) that pull the valve together; the muscle or muscles that withdraw the foot, called the retractor pedis, and the protractor pedis that pulls the foot out. Not only are these scars often very distinct in themselves, but we may frequently observe lines running tangentially from their circumferences towards the umbo, to which they all converge. These lines enclose the areas previously occupied by the muscular impressions; in other words, they show the directions in which the muscles named above shifted their positions as the animal grew.
Fig. 129.—Diagram of the Anatomy of a Lamellibranch
f, mouth, with labial palps; g, stomach; i, intestine, surrounded by the liver; a, anus; r, posterior adductor muscle; e, anterior adductor muscle; c, heart; d, nerve ganglion; m, mantle (the right lobe has been removed); s, siphons; h, gills; ft, foot
Now let us obtain a few species of live lamellibranchs, put them in a vessel of sea water, and observe them after they have been left undisturbed for a time. The shell will be seen to gape slightly, exposing the edges of the two lobes of the mantle which lie closely on the inner surface of the valves, thus completely enveloping the body of the animal; and at one end, usually the narrower end in the case of irregular shells, we shall observe two openings—the siphons, sometimes enclosed within a tube formed by a prolongation of the united mantle lobes, and protruding from between the valves, and sometimes formed by the mere contact of the mantle lobes at two adjacent points. If now we introduce a little carmine or other colouring matter by means of a glass tube, setting it free near the lower siphon—the one more remote from the umbo of the shell, we observe that it enters the body of the mollusc through this opening, and reappears shortly afterwards through the upper or dorsal siphon. Thus we see that water currents are incessantly circulating in the body of the animal, entering by the inhalent or ventral siphon, and leaving by the exhalent or dorsal siphon. These currents are maintained by the vibratile action of thousands of minute cilia belonging to cells that line the cavities of the body, and serve to supply the animal with both air and food; for lamellibranchs, being gill-breathers, derive the oxygen necessary for respiration from the air held in solution by the water, and their food consists entirely of the minute living creatures that always abound in natural waters.
Again, we shall find that some of our live bivalves have protruded a thick, conical, fleshy mass—the foot, from the opposite end of the body. This organ is the means of locomotion in the case of the burrowing and other free-moving bivalves, but is developed to a less extent in those species that lead a sedentary life. Thus, the common Edible Mussel secretes a tuft of strong silky fibres (byssus) by means of which it fixes itself to a rock or other body, and therefore does not need the assistance of a muscular foot; and an examination of its body will show that the foot is very small in proportion to the size of the animal, as compared with that of the wandering and burrowing species. The same is true of the oyster, which lies fixed on its side, the lower valve being attached to the surface on which it rests.
Fig. 130.—Mytilus edulis, with Byssus