Polyplacophora (Loricata; chitons, coat-of-mail shells; phylum Mollusca, class Amphineura) A subclass of amphineurans in which seven or eight dorsal plates are generally composed of calcium carbonate and are enclosed by a spiculate girdle. The plates articulate with one another and overlap to a varying extent. The anterior and posterior plates differ from the others, the anterior plate often being ribbed. The head is often reduced. It contains a central mouth with a radula which most chitons use to scrape algae from rocks. The gills, which may be numerous, lie in a groove between the ventral foot and the girdle. The anatomy is typically molluscan, and the primitive nervous system has ganglionic swellings. Polyplacophora are entirely marine, often occurring interstitially beneath rocks, and first appeared in the Upper Cambrian.

Polyplacophora (chitons; phylum Mollusca) Subclass of Amphineura (or in some classifications ranked as a molluscan class) in which seven or eight dorsal plates, generally composed of calcium carbonate, are enclosed by a marginal girdle. The plates articulate with one another and overlap to a varying extent. The anterior and posterior plates differ from the others, the anterior plate often being ribbed. Polyplacophora are entirely marine, and first appeared in the Upper Cambrian.