The visceral mass is enclosed by the horseshoe-shaped pedal-retractor muscle. After it has oxygenated organs in the visceral mass, the colourless blood, depleted of oxygen, collects in visceral sinuses. It then passes in vessels through gaps in the pedal-retractor muscle (1) into an afferent branchial vessel that runs through the mantle skirt around the edge of the distal face of the muscle (not visible, 2 marks position). The afferent branchial vessel distributes blood to all the gill leaflets (3) which reoxygenate it as it passes through them. The oxygenated blood passes into a large efferent pallial vessel (a.k.a. efferent branchial vessel) and passes forwards on both right (4) and left (5) sides of the animal. The
flow of blood in the efferent pallial vessel (e.p.v.) on the right (4) continues round the front of the head to join the left e.p.v. in a trunk (6) that enters the left side of the nuchal cavity to connect with the heart positioned behind the left of the cavity.
Concise
Part 1, SHELL FORMS: 02 Patella pellucida. Growth Stage 2 (GS2). July 2012. Littoral. North Yorkshire, England. © P. Lightfoot.
Part 2, BODY & ANATOMY: 03 Patella pellucida. Growth Stage 3 (GS3). 2009. Sublittoral, Farne Islands, Northumberland, England. © P.Lightfoot.
Part, 3 HABITS & ECOLOGY: 04 Patella pellucida. Length 4.6mm. Growth Stage 3 (GS3). September 2016. Yorkshire, England. © P.Lightfoot.