This slide shows a section of skin from a shark containing
numerous dermal placoid scales. Although the skin may feel rough to the human touch, the backward orientation of the
spines on these scales actually reduces hydrodynamic drag in water. In sharks, placoid scales are modified anteriorly
to form replaceable rows of teeth in both jaws, and it is thought that these scales are homologous to the teeth of other vertebrates.
This slide shows a group of ganoid scales on a gar (a predatory
freshwater fish). Ganoid scales are thick and non-overlapping and are composed of bone overlaid with an enamel-like
substance called ganoin. This type of scale is also seen in the lobed-finned coelacanth.
This slide shows a cycloid scale. Cycloid scales
are thin and overlapping, permitting more flexibility. Unlike ganoid scales, cycloid scales grow as the fish grows,
and in some species, show annual growth rings. Cycloid scales are found
This slide shows a typical ctenoid scale. Ctenoid
scales are essentially cycloid scales with teeth at their posterior edges. It is thought that these teeth help to reduce
hydrodynamic drag during swimming.