The mollusk or molluscs is a large phylum(taxonomic rank below Kingdom and above Class) of invertebrate animals.
The two most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, and the organization of the nervous system.
The gastropods (snails and slugs) include by far the most classified species, accounting for 80% of the total. Cephalopod molluscs such as squid, cuttlefish and octopus are among the most neurologically advanced invertebrates. Either the giant squid or the colossal squid is the largest known species of animal without a backbone.
Sources: http://en.wikipedia.org/wiki/Mollusca
1.The two most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion.
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3.Molluscs have more varied forms than any other animal phylum.
source:http://en.wikipedia.org/wiki/Mollusca