Tunicate

Ascidiacea (2,300 species)
Thaliacea
Appendicularia
Sorberacea

Urochordata (sometimes known as tunicata and commonly called urochordates, tunicates, sea squirts) is the subphylum of saclike filter feeders with incurrent and excurrent siphons. There are also a few exceptions to this plan, like the predatory Megalodicopia hians, looking something like a cross between a jellyfish and a Venus Flytrap. They are members of the phylum Chordata, which also includes lancelets and all vertebrates including humans. They are now considered as the closest relatives to craniates (i.e. hagfish and vertebrates), having dethroned lancelets from that position. As with other chordates, tunicates possess a notochord during their early stages of development.

They lack segmentation, even in the tail. Metanephridia are absent. The original coelom is degenerated to a pericardial cavity and gonads. Except for the pharynx, heart and gonads, the organs are enclosed in a membrane called an epicardium, which is surrounded by a jelly like matrix known as mesenchyme. The motile larval stages may have the appearance of a tadpole, whereas the adult stage has a barrel-like, sedentary form. They feed by filtering sea water through a gill basket.