Initially this group was divided into those that lived solitarily and those that lived colonially. Now considered as a single class the Ascidians are typical tunicates often referred to as sea squirts. Their developmental forms (11 – 13) maintain the four main characteristics of chordates (blue; dorsal hollow nerve chord, gold; notochord, pharyngeal gill slits and post anal tail both not represented here) however as an adult many of these have been reduced or lost to suit their sedentary, filter feeding niche. The large majority of space is taken up by the pharynx (4 & 10) which is used to filter particulate organic matter from water moved through the siphons by cilia. Ascidians have a closed circulatory system (2, red) with a heart that can switch direction. Gametes and feaces are transported from the base of the adult to the outside of the pharynx (10, r & a) where they are carried by water currents out of the ex-current siphon.
Anatomy of members of the Class Thaliacea showing: internal structures through transluscent bodies, variation of individual body plans (1, 3, 4, 6, 7 & 8′) and arrangements of colonies (5 & 10). The general body plan is of seiving structures held within a tubular pharynx where water is funnelled for food, respiration and propulsion. Note the muscle banding used to create water flow and the centralisation of nervous tissue (labelled N) which is indicative of their place within the Chordata.
Comparison of the two tunicate classes Appendicularia and Ascidiacea showing morphological features through-out development. Note that the class Ascidiacea is paraphyletic and has been reclassified as other groups within Tunicata.