Ascidians belong to the subphylum Urochordata - one of the major groups of the phylum Chordata, which includes the vertebrates (fishes, amphibians, reptiles, birds and mammals). Although you don't look much like a sea squirt now, during development before you were born, you had the same characteristics present in all chordates at some stage of their life
These characteristics of chordates include:
- a nerve cord along the back of the body
- a 'notochord' or firm rod of cells beneath the nerve cord (this is your backbone)
- gill slits (they have disappeared now in some chordates, but were present during evolutionary development).
Ascidians are an evolutionary link between invertebrates and vertebrates. They have a primitive backbone at some stage of their life cycle, but in other aspects they resemble invertebrates.
Most ascidians are hermaphrodites (produce both eggs and sperm) and reproduce by external fertilisation (releasing eggs and sperm into the water). The free-swimming larva they produce are known as ascidian tadpoles. After a few hours, the 'tadpoles' secrete slime, and attach themselves to a rock surface head-first, and then absorb their tail. Adult ascidians are 'sessile' (unable to move around) and filter food particles from the water by pumping water in one siphon and out the other.
Their common name of sea squirt arises from their habit of squirting a jet of water when you stand on or near them when they are uncovered at low tide.