The Ascidiacea constitute a class of the subphylum Tunicata or Urochordata, which are chordate animals related to the vertebrates. Together with the subphylum Cephalochordata (with the Amphioxus, Branchiostoma lanceolatum), the Tunicata form the phylum Chordata. This phylum lacks a backbone but possesses the three distinguishing chordate characteristics. At some time in the life cycle a notochord, a dorsal hollow nerve cord and pharyngeal clefts (called stigmata) are present (tunicate larva).
The Tunicata consist of three classes:
- Ascidiacea- Sea-squirts
- Larvacea (Appendicularia)
- Thaliacea- Salps
These last two classes are specialized for a free-swimming planktonic existence and are not treated here.