7TH GRADE SCIENCE

FISH SCALES

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Placoid scales

Lab_9b-01a
FredsLine

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.

Ganoid scales

Lab_9b-02a
FredsLine

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.

Cycloid scales

Lab_9b-03a
FredsLine

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

Ctenoid scales

Lab_9b-04a
FredsLine

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. 

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