tunicate ascidian WIKIPEDIA:Ascidiacea Ascidiacea (commonly known as the ascidians or sea squirts) is a class in the Tunicata subphylum of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic"made of the polysaccharide tunicin, as compared to other tunicates which are less rigid. ascidiacea sea squirts requires discussion