Gills in right pallial cavity/groove of a 42 mm long specimen. Dorset, England. April 2017.
Each gill has an axis with many lamellae on either side. This resembles the usual molluscan ctenidium, but it may be an independantly evolved feature on chitons.
Oxygen-bearing inhalent water (blue) passes between the lamellae from the anterior to posterior, and blood (red in image but actually non-red haemolymph) counterflows within the lamellae from posterior to anterior. Oxygen-depleted blood enters the gill through a branch of the afferent branchial sinus and leaves, having acquired oxygen in the lamellae, via the efferent branchial sinus.

KEY ID FEATURES : 01 Acanthochitona fascicularis