Placoid scales are small, tooth-like structures found on the skin of certain fish, particularly in the class Chondrichthyes, which includes sharks and rays. These scales are composed of a hard outer layer of enamel and a core of dentin, similar to human teeth. They provide protection and reduce drag as the fish swims through water. Unlike the overlapping scales seen in bony fish, placoid scales are embedded in the skin and have a rough texture. This unique structure helps to protect the fish from predators and parasites while also contributing to their streamlined shape, enhancing their swimming efficiency.