. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. SHELL. 565 posed of thin laminae placed side by side, which separate from one another in the planes of cleavage when the shell is fractured. As first pointed out, however, by Mr. Bowerbank, each of these laminae really consists of a series of cells in close apposition ; and the plates are disposed alternately in contrary directions, so that each series of cells intersects the one beneath it nearly at right angles, as seen in fig. 420. Although the intimate structure of Fiq. art ion of fractured surface of middle l
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. SHELL. 565 posed of thin laminae placed side by side, which separate from one another in the planes of cleavage when the shell is fractured. As first pointed out, however, by Mr. Bowerbank, each of these laminae really consists of a series of cells in close apposition ; and the plates are disposed alternately in contrary directions, so that each series of cells intersects the one beneath it nearly at right angles, as seen in fig. 420. Although the intimate structure of Fiq. art ion of fractured surface of middle layer of Cyprcta mauritiana, showing laminae, composed of prismatic cells obliquely crossing one another. Magnified 235 diameters. (After Bowerbank.) each of the three layers of the shell is essen- tially the same, yet the disposition of the laminae is not the same in any two adjoining ones, — an arrangement which adds greatly to the strength of the shell. The planes of the laminae are always as nearly as possible either parallel or at right angles to the lines of growth ; those of the inner and outer layers always having the same direction with each other, but those of the middle layer being set at right angles to them. When, therefore, a section is made parallel to the surface of the shell, it will cut the edges of the laminae of which the layers traversed by it are composed; but if the section be made in a direction per- pendicular to the surface, and pass through the middle layer in the plane of its laminae, it will cut through the edges of the laminae making up the interior and exterior layers; whilst if the section traverse the two latter in the plane of their laminae, it will cut across the laminas of the middle layer. The principal departures from this plan of structure are seen in Patella, Chiton, Ha- liotis, and Turbo and its allies. In Patella, the inner and outer layers are composed of large and irregular laminae, by no means firmly adherent to one another; but th
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