Note 323 — Chapter VIII · THE DIGESTIVE ORGANS, JAW, AND RADULA: EXCRETORY ORGANS
Reading edition
The substance both of the jaw and radula is neither crystalline nor cellular, but laminated. Chitin is the substance which forms the ligament in bivalves, the ‘pen’ in certain Cephalopoda, and the operculum in many univalves. Neither silica nor keratine enter into the composition of the radula.