Female has a ventral pedal gland with an intense yellow opening (1) in the anterior half of the sole.
The foot folds up to position the ventral pedal gland (2) on a leading edge so it can place and shape an egg-capsule. The gland drives the egg-capsule into a cavity bitten into a compound ascidian and moulds the final shape of capsule.
For further detail, see 21 Trivia arctica &  21.1 Trivia arctica. and  25 Trivia arctica

Height (longest shell-dimension) 9.4mm. Menai Strait, Wales. March 2014.