Tunicata are sessile or free swimming animals, either solitary or colonial. They have an oral and atrial (cloacal) siphon that are located either opposite (in the Class Thaliacea and Class Appendicularia) or besides each other (in the Class Ascidiacea). Tunicates are bilaterally symmetrical, at least during the early developmental stages. They possess a pharynx with gill slits, a dorsal neural tube, and a notochord that disappears with development (not in the Appendicularia). Ascidicea and Thaliacea construct a body wall or test of celluloid tunicine, that is absent in the Appendicularia; these build a delicate gelatinous house (that is not homologous to the test).
In general, the Tunicata have a rather small (ca. 1 mm) planktonic larval stage. The larval development may be suppressed, as is the case in some species of the Ascidiacea, or absent as in the Appendicularia.
The Tunicata comprise three classes with some 3000 species:
¥ Class Ascidiacea (or sea squirts), consisting of sessile filter-feeding species, which have a planktonic larva, unless there is a suppressed larval development.
¥ Class Thaliacea (or salps), exclusively holoplanktonic,[t] with about 73 species.
¥ Class Appendicularia (or larvacea) exclusively holoplanktonic,[t] with about 70 species.
The key to pelagic tunicates of the North Sea starts at Page 454: Tunicata. The following species are included: