Length 26 mm. Ventral view. Llŷn, North Wales. March 2016.
Narrow mantle cavity (1) runs around whole animal. Fifteen ctenidia (2) on each side in posterior half of mantle cavity (merobranch arrangement). Fewer ctenidia (gills) on smaller specimens.
Mantle fold (3), with extended sole, can seal the mantle cavity into a tube that conducts inhalent water from anterior through ctenidia for respiration, and past gonopores, nephridiopores and anus (4) to carry away reproductive and waste materials in an exhalent current through gap created by deflection of hyponotum (5) at posterior.
Water-current created by movement of cilia on ctenidia and mantle. Water enters pallial cavity wherever girdle is raised at anterior. Adjacent ctenidia have interlocking cilia so all work as a unit (as in many bivalves).

KEY ID FEATURES and similar species at: 01 Acanthochitona crinita