Parameter Tree:
EARTH SCIENCE > BIOLOGICAL CLASSIFICATION > ANIMALS/INVERTEBRATES > MOLLUSKS

Definition:
Mollusks are fundamentally bilaterally symmetrical coelomates with
segmentation almost or completely absent; muscular foot, typically
ventral; head usually with concentrated sense organs; dorsal mantle of
tissue folded to form a cavity with usually houses gills; well developed
blood vascular system and usually an internal or external calcareous
shell. 


Reference:
Burn, David M. Animal World. 1980. Octopus Books Ltd. London.