The shell of the wing-shells, Pinna ingens ([Plate XVIII]., No. 7), is composed of hexagonal cells, filled with partially translucent calcareous matter, the outer layer of which can be split up into prism-like columns. Figs. 3 and 6 are horizontal sections of the Haliotis splendens, with stellate pigment in a portion of the section, and wavy lines, as in the dentine of the human tooth, and of Terebratulata rubicuna, showing radiating perforations. Nos. 4 and 5, sections of the shell of a crab, show pigment granules beneath the articular layer and the general hexagonal structure of the next layer.
Some difference of opinion has been expressed with regard to the formation of pearls, but it is now generally understood to be a diseased condition. Pearls are matured on a nucleus, consisting of the same matter as that from which the new layers of shell proceed at the edge of the mussel or oyster. The finest kinds are formed in the body of the animal, or originate in the pearly-looking part of the shell. It is from the size, roundness, and brilliancy of pearls that their value is estimated.
The microscope discloses a difference in the structure of pearls: those having a prismatic cellular structure have a brown horny nucleus, surrounded by small imperfectly-formed prismatic cells; there is also a ring of horny matter, followed by other prisms, and so on, as represented in [Fig. 374]; and all transverse sections of pearls from oysters show the same successive rings of growth or deposit.
Fig. 374.
1. A transverse section of a Pearl from Oyster, showing its prismatic structure 2. A transverse section of another Pearl, showing its central cellular structure, with outside rings of true pearly matter. (Magnified 50 diameters.)
In a segment of a transverse section of a small purple pearl from a species of Mytilus ([Fig. 375]), all trace of prismatic structure has disappeared, and only a series of fine curved or radiating lines is seen. This pearl consists of a beautiful purple-coloured series of regular laminæ, many of which have a series of concentric zones, and are of a yellow tint. The most beautiful sections for microscopic examination are obtained from Scotch pearls.
Preparation of the Teeth and Shell of Mollusca for Microscopical Examination.—The method of preparing lingual membranes of Mollusca is as follows: Under a dissecting microscope, and with a large bull’s eye lens, cut open and expose to view the floor of the mouth; pin back the cut edges throughout its length, and work out the dental band with knife and forceps. The band being detached, place it in a watch-glass, and boil in caustic potash solution for a few minutes. Having by this process freed the tongue from its integuments, remove it, wash it well, and place it for a short time in a dilute acid solution, either acetic or hydrochloric. Wash it well and float it upon a slide; with a fine sable brush open it out flat, and remove whatever dirt or fibre may be adhering to it. Lastly, place it in weak spirit and water, and there let it remain for a few days before mounting in formalin. Canada balsam renders them rather too pellucid, and the finer teeth are thereby lost.
Fig. 375.