Adaptations specific to the habitat of the gastropod or bivalve are often reflected in the shell. Factors like predation pressure, substrate, grain size, turbidity, behaviour, etc. have an effect on the morphology of the shell. As described above, a thick shell is usually a good defence against predators, but this is not always feasible because of the demands imposed by the inhabited environment. An animal living in soft sediments with a low density, (specific gravity), may sink if its own bulk density exceeds that of