Fig. 10.
SEM micrograph of worn mineralized tooth of C. olivaceus. Note the sharp wedge angle and the fact that the crystal units in the leading part are exposed over their entire length. The radula was cleaned in a trypsin-containing solution. Bar=10 μm.
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SEM micrograph of two worn mineralized unicuspid teeth of P. vulgata. Note the ...
Fig. 11.
SEM micrograph of two worn mineralized unicuspid teeth of P. vulgata. Note the sharpness of the wedge angle. The radula was cleaned in a trypsin-containing solution. Bar=10 μm.
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The orientations of the surfaces of wear depicted in Fig. 9 are coincident with orientations imposed by the internal structure of the tooth as illustrated in Fig. 2. In C. olivaceus the surface of wear always occurs along the elongation of crystal units and fibres in both the leading and trailing part (cf. also Fig. 10); in P. vulgata the surface of wear is oriented parallel to the acicular goethite crystals in the trailing part of the tooth. Obviously, the fractures arising during wear preferentially propagate along the directions provided by the internal structure of the tooth. This conclusion was also reached from a detailed ultrastructural study of worn teeth of both C. olivaceus [23]and P. vulgata [22].
High-speed film recordings of the radular movement of actively grazing limpets and chitons have shown that the tooth cusp is always pressed against the substrate in an upright position as depicted in Fig. 12 (Videler et al., in preparation). In this schematic figure, which represents a transverse section of a limpet tooth, also the internal structure is drawn from which the orientation of the subsequent surfaces of wear can be inferred. The figure shows that (1) the wedge angles are always sharp, (2) the clearance angles are always positive throughout the working-life of a tooth, and (3) rake angles are positive in the first stages and may become negative in the latest stages of the working life of a tooth.
Schematic drawing of limpet mineralized tooth in grazing position. In this ...
Fig. 12.
Schematic drawing of limpet mineralized tooth in grazing position. In this sagittal section of the cusp the orientations of the constructive elements are shown. Also shown are clearance, wedge and rake angles.