A salp is a barrel-shaped, planktonic tunicate. It moves by contracting, thus pumping water through its gelatinous body. This is also used for locomotion, and salp jet propulsion is one of the most efficient in the animal kingdom. The salp strains the pumped water through its internal feeding filters, feeding on phytoplankton. Salps appear to have a form preliminary to vertebrates, and are used as a starting point in models of how vertebrates evolved. Scientists speculate that the tiny groups of nerves in salps are one of the first instances of a primitive nervous system, which eventually evolved into the more complex central nervous systems of vertebrates.