J.E. Gray, 1821
The Polyplacophora are a group of entirely marine molluscs commonly known as chitons or 'coat-of-mail' shells. They are an ancient group, isolated valves having been found in the Upper Cambrian, but the fossil record is generally poor.
— Adult Polyplacophora are bilaterally symmetrical with an ovoid body that is very greatly flattened dorso-ventrally. The mantle bears eight transverse shell valves on its dorsal surface. The mantle is developed into a thickened girdle that can bear spines and/or bristles, and can overgrow the shell plates. Head poorly developed, covered by anterior girdle. Eyes and sensory tentacles lacking. Radula large. Large, muscular foot makes up most of ventral surface. Six to 88 pairs of bipectinate ctenidia.
The biology of the North Sea species is poorly known largely because they have tended to be overlooked, being for the most part small in size, somewhat cryptic and predominantly sub-littoral in habit.
— Larva. For the present zooplankton key, only the larval stages are of interest. However, the knowledge at the species level is insufficient and the larvae are not further keyed out. The chiton larva is typically a trochophore, there is no veliger present; by longitudinal growth and loss of the ciliar crests of the trochophore larva, the adult chiton develops.
The following species of Polyplacophora occur in the area (De Kluijver et al., 2000a):
Acanthochitona crinita
Acanthochitona fascicularis
Hanleya hanleyi
Callochiton septemvalvis
Ischnochiton albus
Lepidochitona cinerea
Tonicella marmoreus
Tonicella rubra
Leptochiton asellus
Leptochiton cancellatus