Polyplacophora

Polyplacophora, commonly known as chitons, are marine mollusks with a dorsoventrally flattened body protected by eight overlapping calcareous plates. These eight plates form a dorsal shield, while a broad ventral foot enables the animal to cling to hard substrates. A flexible girdle surrounds the plates and often bears ornamentation. Chitons vary in size and color and are typically found on rocky substrates in coastal waters.

Anatomy and feeding: Chitons lack a distinct head and have a simple nervous system. Their radula bears rows of iron oxide–reinforced teeth used to scrape algae and biofilm from rocks. The mantle margins border the girdle and may host sensory cells in some species. Most chitons are herbivores or omnivores, grazing on encrusting algae, diatoms, and detritus on hard substrates.

Distribution and habitat: Chitons occur worldwide, from cold to tropical seas, in shallow to moderately deep water. They prefer exposed rocky shores and other hard substrates where they can attach firmly, often in crevices or under slabs. The group is especially diverse along temperate coastlines.

Life cycle and development: Reproduction is typically sexual with external fertilization. Eggs and sperm are released into the water column, and development includes free-swimming larval stages common to mollusks before settling as eight-plated juveniles.

Evolution and taxonomy: The Polyplacophora have a fossil record dating back to the Cambrian. The eight-plated shell is a distinctive feature that persists in many lineages, and chitons show considerable diversity in plate morphology and radular structure. Modern chitons inhabit several orders and families within Mollusca.