Often the term “nudibranch” is used for all sea slugs. However, while all nudibranchs are sea slugs, not all sea slugs are nudibranchs. The sea slug featured in the video below is the rarely seen Philinopsis troubridgensis. This species belongs to the "headshield slug" group.
The vast majority of headshield slugs still possess external shells, though generally they are unable to retreat into them fully, whereas others have reduced or internal shells. They all have well evolved headshield, which they use to dig under the sand. These slugs also have highly sensitive eyes on the anterior end of the head and bunched cilia around the mouth, which is used to track their prey's mucous trail.