The periostracal groove of the deep-sea hydrothermal vent Mytilid presents a particular anatomical structure. The initial pellicle derives from pre-periostracal membranes surrounding a granular material. The individualized pellicle then progresses between the generating cells where it is coated until its extrusion in the growing periostracal groove. After extrusion, the periostracal pellicle is regularly coated by material secreted by the microvilli of the adjacent periostracal groove cells.