Chordata are animals which in some stage of their life have gill slits and a skeletal axis known as a notochord.
Subphylum Tunicata. These are primitive chordates; of about 700 species, all are marine.
Class Larvacea (Appendicularia). These are small planktonic forms, sometimes abundant. Examples: Oikopleura (fig. 228e), Fritillaria.
Class Ascidiacea. These are sessile ascidians such as Ciona and Culeolus.
Class Thaliacea. This class is made up of pelagic tunicates that float singly or in chains; they may be very abundant at the surface in the warmer waters. Examples: Salpa, Doliolum.
Other protochordates are the wormlike Enteropneusta and the fishlike Cephalochorda, both of which are found burrowing in mud and sand.