. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. ciion of shell of Unio Occident, near the Up, showing the arrangement of the outet or prismatic, internal or nacreous layers: a a', bV, c</, successive Hues of growth; d, margin of the valve Vertical section and the Magnified 8 diameters. represents a section of one of the valves of Unio occiclens, taken perpendicularly to its surface, and passing from the margin (at the right hand of the figure) towards the umbo (which would be at some distance beyond the left). This section brings into view the two substances of whi
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. ciion of shell of Unio Occident, near the Up, showing the arrangement of the outet or prismatic, internal or nacreous layers: a a', bV, c</, successive Hues of growth; d, margin of the valve Vertical section and the Magnified 8 diameters. represents a section of one of the valves of Unio occiclens, taken perpendicularly to its surface, and passing from the margin (at the right hand of the figure) towards the umbo (which would be at some distance beyond the left). This section brings into view the two substances of which the shell is composed ; traversing the outer or prismatic layer in the direction of the length of its cells, and passing through the nacreous lining, which is seen to be made up of numerous laminae, separated by the lines aa\ bb', cc', &c. These lines evidently indicate the successive formations of this layer; and it may be easily shown, by tracing them towards the umbo on the one side, and towards the margin on the other, that at every enlargement of the shell its whole interior is lined by a new nacreous lamina, in immediate contact with that which preceded it. The number of such laminae, therefore, in the oldest part of the shell, in- dicates the number of enlargements which it has undergone. The outer or prismatic layer of the growing shell, on the other hand, is only formed where the new structure projects be- yond the margin of the old ; and thus we do not find one layer of it overlapping another, except at the lines of junction of two distinct formations. When the shell has attained its full dimensions, however, new laminae of both layers still continue to be added ; and thus the lip becomes thickened by successive for- mations of prismatic structure, each being applied to the inner surface of the preceding, instead of to its free margin. The same ar- rangement may be well seen in the Oyster; with this difference, that the successive layers have but a comparative
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Keywords: ., bo, booksubjectanatomy, booksubjectphysiology, booksubjectzoology
