Mollusca

In the marine environment, of of the advanced invertebrates groups are the mollusca. They are soft bodied creatures and have over 80,000 species. Some species include snails, clams, muscles, oysters, squid and octopus. There are 8 classes within the molussca phylum and three of the main ones include the gastropods (snails), bivalvs (clams, muscles and oysters) and chepalopods (octopus, squid and cattle fish). They are all very different species but they all have the same stomach and digestive tract, which is why they are grouped together in the same phylum. The gastropods are the largest class of mollusca, containing about 80% of the species. They live in shells, which provide protect against predators. Snails can be found in freshwater, marine and terrestrial environments. They are herbivores. The class Bivalvia, contains two shells and are, for the most part, symmetrical. There are over 30,000 species, including clams, muscles, oysters and scallops. They can live in both the marine and freshwater environments. The feed by filtering water around them. Chepalopods contain a bilateral body symmetry. It is estimated that there are about 1000-1200 species in the Chepalopod class. They have a large head and usually 8 arms or tentacles.

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~http://www-biol.paisley.ac.uk/Courses/Tatner/biomedia/units/moll1.htm
~http://en.wikipedia.org/wiki/Mollusc