The Celtic sea slug has a solid, oval and fleshy body up to 13 mm in length and 6 mm in width. It is a dark green-black inconspicuous slug with a mantle that is covered in large, coarse, evenly spaced tubercles, to which sand grains often adhere. When Onchidella celtica is motionless, the mantle completely obscures the head and pale grey foot. The head bears two short, thick, cylindrical tentacles with eyes at the tips. The head is only visible when the Celtic sea slug is mobile.This species is not an opisthobranch sea slug. It is a pulmonate slug more adapted to terrestrial and freshwater habitats, and related to most land snails. Being a pulmonate gastropod means that it has no gills. Therefore the mantle cavity acts as a lung and is closed by a sphinctered opening called the pneumostome, situated close to the anus. Pulmonate sea slugs also have no shell. It has a veliger larval stage which is completed within the egg capsule.