Final answer:
Adult larvaceans lack a notochord, dorsal hollow nerve cord, and post-an/al tail due to the presence of a cellulose-like tunic, although they retain pharyngeal slits and an endostyle.
Step-by-step explanation:
Larvaceans, which are a group of Urochordata, are known for having a cellulose-like tunic covering their outer body. As larvaceans are classified as chordates, their adult forms lack certain features typical of the phylum. Specifically, because they have the cellulose tunic, adult larvaceans do not have a notochord, a dorsal hollow nerve cord, or a post-an/al tail, which are key characteristics of chordates.
However, they do retain pharyngeal slits and an endostyle. It's important to note that the larval form of the tunicate does have all the typical chordate features before undergoing metamorphosis into the adult form, at which point these features are lost.