polyplacophorans

Polyplacophorans, commonly known as chitons, are a class of marine mollusks distinguished by a dorsoventrally flattened body and eight articulating calcareous plates that form a protective shell along the dorsal surface. These plates are embedded in a tough, flexible girdle that borders the animal’s margins. The ventral side bears a broad, muscular foot used for creeping on rocky substrates.

The anatomy includes a mantle cavity with numerous gill filaments (ctenidia) along the mantle edge, and a radula—an extended ribbon bearing rows of mineral-reinforced teeth used to scrape algae from rocks. The shell plates provide protection while allowing flexibility; the plates slide relative to each other as the animal moves. A simple, or otherwise reduced, eyespot system may occur in some species.

Distribution and ecology: chitons occur in oceans worldwide, from polar to tropical regions, primarily on hard substrates such as rocks in intertidal to shallow subtidal zones. They are mostly herbivores or detritivores, feeding on biofilm and microalgae scraped from the rock surface. Their shell plates and girdle patterns show considerable diversity, often reflecting habitat and phylogeny.

Reproduction and development: most chitons are gonochoric with external fertilization; eggs and sperm are released into the water column. Early development includes free-swimming larval stages typical of mollusks, such as trochophore and subsequent veliger stages, before settling as juvenile chitons.

Fossil and significance: the polyplacophoran fossil record extends from the Cambrian to the present, illustrating one of the earliest and most conservative molluscan body plans. Modern chitons comprise roughly 900 described species, with diversity concentrated in shell plate arrangement and girdle ornamentation.