Mollusca
. Mollusk is a common name for members of a phylum of soft-bodied animals (Latin mollus, “soft”), usually with a hard external shell. The mollusks represent a diverse group of marine, freshwater, and terrestrial invertebrates, including such varied forms as snails , chitons, limpets, clams , mussels, oysters , octopuses , squid , cuttlefish, tusk shells, slu gs, nudibranchs, and several highly modified deep-sea forms. They all have one anatomical feature in common, the presence of a shell at some stage in the life cycle. Although most mollusks have a shell as adults, the octopus, squid, and deep-sea forms do not. They do however have a small, shell-like structure, called a shell gland, present for a short time during embryonic development. The mollusk phylum is the second largest in the animal kingdom, after the arthropods. Scientific classification: Mollusks make up the phylum Mollusca. In the class Aplacophora, the body is wormlike. No shell exists, only a tough mantle, and the foot...