|One of many species of lancelet. Lancelets are also known as amphioxis.|
As I already mentioned, these animals are rather simple, especially when compared to most other chordates. They have only a small brain, but some other parts of their anatomy are fairly sophisticated. They are filter feeders and have a special ciliated "wheel organ" used to bring water into their mouths for this exact purpose. The digestive system is fairly straightforward. They do have a well developed circulatory system and a simply excretory system made up of nephridia (simply, rudimentary kidneys).
The fossil history of lancelets is very sparse due to their soft-bodied nature. As a rule, fossils are only formed of hard parts of the body. It is quite rare for a fossil to form in the right sort of way and preserve a soft-bodied creature. However, just such an event occurred to produce the famous Burgess Shale. This fossil deposit shows example of what are likely lanelets, which means that these creatures have been around since at least the Cambrian period. Today, lancelets are quite common, with concentrations in some areas being as high as five thousand in a square meter of sand. They are eaten in some areas of the world.
Source is the University of California Museum of Paleontology. Image is from Wikimedia Commons under a Creative Commons license.