Mollusks have bilateral symmetry, although some mollusks have lost their symmetry during development, as reported by Rutger's University. The mollusk phylum include organisms like snails, oysters, claims, octopuses and even squids. Most have a calcareous shell, whether it be internally or externally, and well-developed respiratory, digestive, excretory and circulatory systems.
Mollusks are defined by the fact that all of these organisms have three distinctive anatomical body parts: a head-foot, a visceral mass and a mantle. The head-foot is a muscle that is responsible for sensory detection and locomotion. The visceral mass is composed of all of the vital organs, and the mantle is a hard, calcareous shell that covers and protects the visceral mass from predators and hazards.
The phylum of mollusks can be further separated and divided into four distinctive classes: bivalvia, polyplacophora, gastropoda and cephalopoda. Organisms that fall into the class bivalvia have hinged shells with both right and left halves while organisms that fall into the class polyplacophora have elliptical bodies that are made up of eight plates. The class gastropoda contains organisms that have a visceral mass contained in a spiral, coiled shell and the class cephalopoda contains organisms that exhibit prominent heads and anywhere from eight to ten limbs that extend outwards from the oral region.