The shell is composed of eight overlapping plates or valves. These are joined to each other on the outer margin and undersides to the girdle, a thickened part of the mantle.
Scales, spines or bristles may be present on the girdle.
Since the plates allow flexibility, the animal is able to mold itself to uneven surfaces or roll up in a ball to protect its soft under parts.
The animal has an elongated, bilaterally symmetrical body.
No tentacles or cephalic eyes are present although some species have light-sensitive spots in the integument covering the plates.
The mouth is anterior with a radula.
The anus is posterior.
The muscular foot allows the animal to move easily and to clamp down tightly on the firm substrate.
Approximately 600 living species are known. Most feed by scraping algae off hard surfaces with the radula. Some species prey on small crustaceans.