Practice question

Polyplacophora or chitons are marine molluscs identified by eight dorsal calcareous articulamentum plates overlapping anteroposteriorly, surrounded by muscular girdle often covered by spines or scales, permitting flexible adherence to intertidal rocks and enrollment for protection. Each plate bears insertion plates for muscle attachment. Large flat foot provides powerful suction, broad radula for grazing. Mantle groove houses numerous ctenidia. Single shell characterizes Gastropoda coiled shells, bivalve two shells hinged, cephalopod internal reduced shell. Eight-plate arrangement therefore synapomorphic for Polyplacophora, reflecting segmented shell fields and evolutionary adaptation to rocky wave-swept habitats.