Ascidiacea is a class of marine invertebrates commonly known as sea squirts, belonging to the subphylum Tunicata. Although their adult form is sac-like and simple, ascidians are chordates and possess a notochord and dorsal nerve cord during their free-swimming larval stage. Adults are typically sessile, attached to rocks, docks, mangroves, or other submerged surfaces, and are enclosed in a protective outer covering called a tunic, composed largely of a cellulose-like substance known as tunicin. They feed by drawing water in through an incurrent siphon, filtering out plankton and organic particles with a specialized pharyngeal basket, and expelling the water through an excurrent siphon—hence the name “sea squirt” when water is expelled upon disturbance.