Chitin helicoids accompany protein helicoids in the periostracum of a whelk, Buccinum.

The periostracum of the marine gastropod Buccinum has a helicoidal arrangement of its principal constituent which is a fibrous protein (Hunt and Oates, 1978). Chitin, chemically and physically identified, is present at a concentration of about 6% of the dry weight and can be seen in dispersates of whole periostracum as long fibrils and ribbons between 3 and 14 nm diameter. Deproteinization with hot alkali removes all protein leaving a chitinous 'ghost' of the periostracum. Dispersates, examined negatively stained, show only chitin fibrils and ribbons while sectioned material demonstrates a tenuous, part orthogonal, part helicoidal, architecture based on the chitin residue. The relative roles of the protein and polysaccharide components is speculated upon and comparisons with arthropod cuticle drawn.