The individuals making up the colony are usually less than 1mm long, but colonies can span over one metre. Also known as sea mats as they tend to encrust the surfaces of stones or seaweeds, however there are other colonial forms such as coral like growths, branching plant-like tufts and fleshy lobes.
Ascidians are commonly known as sea squirts. Despite having soft bodies they are not invertebrates but are primitive chordates, the phylum to which back-boned animals belong.
Sea squirts live attached to hard surfaces such as rocks, reefs and shipwrecks, sometimes growing together in clumps with cnidarians and sponges – they can be very colourful. They spend their time filtering seawater, drawing in food-rich water through one siphon (inhalant) and releasing waste water through another (exhalent). Most occur in shallow water where there is plenty of plankton, but there are a few deep-water species.