Shell is a protective skeleton, which forms a characteristic feature of molluscs.
Usually external, but sometimes internal.
Derived from the mantle of the veliger larva.
Structure of Shells
Periostracum: Outermost pigmented layer, made of honey organic substance called Conchiolin.
Secreted by the edge of the mantle.
Prismatic Layer: Also secreted by the mantle edge. Made of crystalline calcareous plates running vertically.
Nacreous or Pearl Layer: Made of Conchiolin and Calcareous plates running alternately and longitudinally.
| Mollusc | Details |
| Chiton | 8 transverse, overlapping calcareous plates, arranged in a longitudinal row. Forms the solid armour covering the dorsal surface. Shell plates are movable on one another, to allow the animal to roll up. Cephalic and Anal (1st and 8th) plates are hemispherical. Others are somewhat rectangular and keeled mid-dorsally. Each plate is made up of two layers. Tegmentum: Upper layer, organic conchiolin matrix. Articulamentum: Lower layer, thicker and denser, made up of Calcium Carbonate only. |
| Dentalium | Shell is external. Cylindrical, tubular, slightly curved, tapering, open at both the ends. During life, shell is buried obliquely in mud, with the wider end lying closest to substrate. |
| Cypraea | Oval, rounded on the top. Shell opening is long and narrow, toothed on both sides and channeled at each end. |
| Mytilus | Pointed in front and rounded behind. Two shell valves are united antero-dorsally by a hinge ligament. Hinge ligament is made up of Conchiolin and is brown, tough, elastic and non-calcareous. Umbo is a whitish knob-like swelling in each valve anteriorly. It is the thickest and oldest portion of the shell. Rings of growth run around Umbo as centre and run parallel to the free margin of the shell. |
| Sepia | Internal shell in cuttle fish. Lies embedded in the upper side, completely enclosed in a sac of mantle. Secreted by its own epithelial lining. Flat, broad and oval in shape, represented by Phragmocone, pro-ostracum (oral end) and rostrum projecting into a spine, at the aboral end. Calcareous matter is arranged in fine parallel layers called septa or laminae enclosing space containing fluid and gas. |