Most runcinids are small (less than 8 mm when crawling) and dark coloured. They are probably quite common but frequently overlooked because of their small size and/or external similarity to acoel flatworms.
They have a smooth, slender and flattened slug-like appearance with a continuous notum, the head shield and visceral mass merging without division, that overhangs all around.
The eye spots are conspicuous dorsally. They possess a gill located near or around the anus often visible at the posterior right.
The foot lacks any kind of tentacular or parapodial expansion but the tail is longer than the notum.
A minute shell remnant may be internal, external or completely lacking.