The shell is whitish, tinted horn to pale brown; partly due to the closely adhering, flimsy periostracum. It has three rows of iridescent blue hyphens, increasing to about eight when 7mm long. The width of the lines remains fairly constant at 0.1mm to 0.2mm at all growth stages.
The body can usually be discerned through the almost transparent shell:
1: two of the muscle-bundles forming the horseshoe-shape pedal-retractor muscle. Blood depleted of oxygen passes in vessels through the gaps between the bundles into the afferent branchial vessel which passes the blood into the gills for oxygenation.
2: whitish pallial gills.
3: colourless efferent branchial vessel (a.k.a. efferent pallial vessel) runs through the mantle around the entire perimeter taking oxygenated blood from the pallial gills to the heart.
4: trunk taking blood from efferent branchial vessel, past the anterior of the pedal-retractor muscle and through the nuchal cavity to the auricle of the heart.

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.