In Lingula the shell is composed of alternate layers of chitin and of phosphate of lime.
They yield chitin in place of chondrin or gelatin - as does also the cartilage of the Cephalopod's endoskeleton.
The parapodia of Chaetopoda are never coated with dense chitin, and are, therefore, never converted into jaws; the primitive " head-lobe " or prostomium persists, and frequently carries eyes and sensory tentacles.
In most cases the organic structure has disappeared, le i tving only a cavity, with perhaps a trace of chitin.
Usually they open into a median vagina, formed by an ectodermal inpushing and lined with chitin.