- Molluscs are invertebrate animals with an unsegmented, basically bilaterally symmetrical
body, generally consisting of head, foot, visceral mass and mantle. The majority of
molluscs live in the sea or in fresh water and use gills for respiration. Certain
terrestrial gastropods breathe by lungs.
It is the characteristic of the molluscs that they have no internal supporting skeleton.
The shell serves to protect the body against the outside world, but it does not provide
support or rigidity. The shell is a product of secretion which by deposition of material
can be thickened, enlarged and if necessary repaired, but it plays no part in the
metabolism of the animal.