D = dorsal view, V = ventral view,
O = outer convex face, I = inner concave face.
Dissection shows that the anterior wall of the buccal cavity is reinforced by a pliable, white, chitinous, antero-dorsal plate, called the “jaw” though it is not articulated and does not bite (1). It serves as an attachment for several muscles and has two lateral wings that meet dorsally at an angle (2) and form an anterior shield for the inner lips (3) when they are open. Within the buccal cavity there is a large pink odontophore, consisting of a right and left bolster (4), which is covered in thick cuticle. The bolsters are strengthened by cartilage-like material.
The anterior of the radula, widened into a hyaline shield (5) , rests on the dorsum of the odontophore and is recessed slightly into the groove (6) between the bolsters. The strong, iron-impregnated, unarticulated teeth are firmly fixed in a backwardly inclined position (7) on the radula, but the anterior tip of the radula bends over the front of the odontophore so that the front row of four teeth are inclined forwards like a chisel (8) . To feed, the strong muscles of the odontophore thrust it forwards against the front of the buccal cavity which, reinforced by the jaw, restrains the odontophore but allows the front teeth to project strongly from the vertex of the gap between the wings of the jaw (9) . When applied to rocky substrate, the teeth easily chisel off the preferred calcareous algae and other growths, and the curve of the withdrawing teeth acts as a scoop to lift particles back to the oesophagus in the buccal cavity. The action is surrounded by the outer lips which prevent the escape of loosened food fragments.
APPENDIX re range advance/retreat: flic.kr/p/2jW74ig