The chitons are another relatively small group, with around 600 species placed in three Orders. They are flattened, elongated animals with eight overlapping dorsal shell plates or "valves," bordered by a thick "girdle" formed from the mantle. The mantle cavity encloses the foot, with which chitons cling to rocky surfaces. Between foot and mantle on each side of the body is a row of gills.
Chitons have a single heart, a pair of kidneys, a simple nervous system comprising two pairs of nerve cords that connect near the mouth, and sense organs including eyes and chemosensory organs.
Chitons are dioecious. Sperm released by males into the water are taken up by females. Fertilized eggs are shed singly or in gelatinous strings. These develop into a trochophore larva; no veliger is present.
Most chitons are small, but one species reaches 30cm in
length. All are marine. Most species live in the rocky intertidal zone,
but some can be found at great depths. They graze on algae, scraping algal
cells
from the substrate with a radula.