why are urochordates called tunicates?

The body wall of urochordates is most of connective tissue like covering called the "tunic".

The urochordates or tunicates are a remarkable group of organisms and typically very beautiful and photogenic. All are marine organisms. The body is enclosed in a more-or-less translucent tunic which consists of living connective tissue. Tunicates are the primitive marine animal having a saclike unsegmented body.  They differ from Cephalochordata and Vertebrata in that the adult form has no notochord, nerve cord, or tail. The immobile adult tunicate looks like a sponge or a mollusk than a chordate.