1) Internal digestive gland.
2) Wide, opaque white, subapical band of pigment.
3) Pointed, unpigmented translucent apex.
4) Base of opaque band often abuts onto red digestive gland.
5) Sometimes, a translucent gap between opaque band and digestive gland.
Left: LWS, Menai Strait, Wales. February 2011. Right: sublittoral. Shetland. January 2021. © B. Arthur.