When these mollusks come into the water, they soon find their transparent white shell too small, and begin to increase its size by means of the mantle, which is an exquisitely sensible fleshy envelope applied to the back of the animal, extending round its sides like a cloak, only meeting in front, and it is for the most part in close contact with the whole interior of the shell. Its edges are fringed with rows of slender tentacles, and studded with glands, which secrete the colours afterwards seen in the shell; the glands in the rest of the mantle secrete only colourless matter.
When the animal begins to enlarge its shell, it attaches the borders of the mantle to the margin of the valves, secretes a film of animal matter, and lines it with a layer of mucus containing carbonate of lime and colour in a soft state, which soon becomes hard, and is then coated internally by the other glands of the mantle with colourless carbonate of lime.
The two strata thus formed, one richly coloured, the other white, often nacreous and brilliantly iridescent, are highly organized substances. Examined with the microscope, they present remarkable varieties in some of the natural groups of bivalve Mollusca; the structure of the Monomyarian Oyster is characteristic of the division which has but one muscle; the Dimyaria, having two muscles, are represented by the Cockle.
Fig. 167. Section of shell of Pinna transversely to the direction of its prisms.
Fig. 168. Membranous basis of the shell of Pinna.
The exterior laminæ at the edge of the fragile valves of a Pinna are often so thin and transparent that the organization of the shells may be seen with a low magnifying power. A fragment has the appearance of a honeycomb on both surfaces ([fig. 167]), whereas its broken edge resembles an assemblage of basaltic columns. The exterior layer of the shell is thus composed of a vast number of nearly uniform prisms, usually approaching to the hexagonal structure, whose lengths form the thickness of the lamina, their extremities its surfaces. When the calcareous part of the lamina is dissolved by dilute acid, a firm membrane is left, which exhibits a hexagonal structure ([fig. 168]), as in the original shell; but it is only in the shells of a few families of bivalves nearly allied to the Pinna that this combination of the organic and mineral elements is seen in this distinct form; it is beautifully displayed in the nacreous shells.
Fig. 169. Section of nacreous lining of the shell of Avicula margaritacea (pearl oyster).