Answer the following question given this paragraph: 

24.2 Subphylum Urochordata The tunicates are located in this subphylum. Along with the subphylum Cephalochordata, these two subphyla make up the invertebrate chordates. Only the tunicate larvae have notochords, nerve cords, and postanal tails. Most adult tunicates are sessile, filter-feeders which retain their pharyngeal slits. Adult tunicates also develop a sac, called a tunic, which gives tunicates their name. Cilia beating within the turnicate cause water to enter the incurrent siphon. The water enters the body, passes through the pharyngeal slits, and leaves the body through the excurrent siphon. Undigested food is removed through the anus. Tunicates are hemaphrodites and can reproduce asexually through budding. In urochordates notochord is confined to larval tail. These lack cranium. These have an open type of circulatory system. Excretion is by neural gland,nephrocytes.


Q: Adult tunicates also develop a sac, called a what?


A:
tunic