Why is cellulose in tunicate tunics unusual among animals?

The presence of cellulose in the tunics of tunicates is considered unusual because cellulose is a glucose polysaccharide that is not normally found in animals.1

While cellulose is the primary structural component of plant cell walls, it is rarely synthesized by members of the animal kingdom. Most animals rely on other structural polysaccharides, such as chitin—which is sometimes referred to as animal cellulose—to form their outer skeletons or protective coverings. The ability of tunicates to secrete a tough tunic containing cellulose distinguishes them from the vast majority of other animals, which typically lack the biological machinery to produce this specific carbohydrate.123