The third group of tube worms is formed by the gillheads ( Cephalobranchiata ), so called, because the softened thread or tree-shaped appendages for breathing are at the head or at least at the anterior segments. They inhabit tubes from which they never emerge voluntarily. Accordingly, the appendages of most segments, except the anterior ones, are much less developed than those of the free-living many-bristles, and the way of life is more peaceful, as evidenced by the absence of dental plates in the oral cavity.