And what does it look like? Quite a lot like a fish. A fish without brain, bone, heart or eyes. So maybe more like a fillet of fish from the shops, just throw it in the frying pan. I guess that's why they are harvested for food in parts of Asia.
They aren't large, reaching about 10 cm at most and species are usually half that. They have a fin running all the way down their back, around the tail and part way along the underside. They can just about swim by wriggling around, but they are much better at digging into the sand of the warm, shallow seas they call home. Once there they can poke their head out and put their cirri to work.
Cirri are the thin tentacle-like strands in front of their mouth. Lancelets use these as sensory organs and to take water in through the mouth while straining out particles too big to eat. Inside there is the pharynx, which is like the throat, with lots and lots of pharyngeal slits. These are much like gills, but they aren't used for breathing like fish. The lancelet can simply breathe through its skin after all. Instead, the gills are covered in a mucus that trap tiny particles of food and move them on for digestion. Meanwhile the water passes through the gills and eventually out through the back end via the atriopore.
The food goes on toward a pouch which has a lining capable of phagocytosis, where food particles are engulfed by cells. This is a bit weird, more common in white blood cells and our very own Amoeba, but it seems that no other chordate uses it in digestion. Oh, and then it all passes out through the anus, which is common amongst chordates but seldom talked about.
Lancelets are separated into males and females, each with a couple dozen gonads. A bit much for chordates. Eggs and sperm are released into the water via the atriopore and the young spend their time drifting in the sea before entering adulthood and diving head first into the mud. Happens to the best of us...