Sometimes know as Tunicata.
The majority of urochordata start life as free swimming larva that exhibit all the characteristics of chordates. Once they settle and attach to a hard substrate they lose their tail and no longer able to move. At which point the majority of their nervous system disintegrates. 
- Ascidiacea - Commonly known as the ascidians or sea squirts 
- Larvacea - Small free-swimming tunicates 
- Thaliacea - Holoplanktonic organisms 
- Sorberacea - Deep-water tunicata