From the Greek plah - plane, plate and eidos - kind, form.
Placoid scales are the most ancient scales preserved in cartilaginous fish (sharks, rays). It consists of a basal plate lying in the skin and a spike sitting on it, covered with enamel. The basis of the scale is dentin, a hard organic substance with calcium salts. The inner cavity of the scale is filled with connective tissue (pulp) with blood vessels and nerve endings. On the body of fish placoid scales are located dnagonal rows. During life, placoid scales undergo constant change. In vertebrate evolution, placoid scales precede the more complexly constructed ganoid scales.
Tags: placoid scales