These shells emerged 600 million years ago. No other class has as many species (more than 50,000) or are as widely collected. They extend from the coastline to the abyssal zone and colonize from fresh water to dry land. Gastropods are characterized by their spiral structures and colors.
Also emerging at the same time as gastropods, these shells all live on seabeds and only reach about an inch in total length.
Emerging 500 million years ago, polyplacophores (aka chitons) live a few inches below the water's surface on reefs. Their shells consist of eight tiny plates layered as shingles with pores. Today there are about 600 species ranging from intertidal areas to the abyssal zone.
About 550 million years ago these shells washed up on beaches around the world. They are small, curved, white tubes formed for specialization purposes. There are 350 species existing today buried in sandy seabeds from the intertidal area to the abyssal zone. There maximum length is five inches and they tend to be white in color.
The first of these shells evolved 425 million years ago during the Cambian or Ordovician period. This class contains approx. 10, 000 species which are mostly marine and freshwater. The largest one is the giant clam and is 4.5 feet long. Bivalves are as their name suggests: two hinged plates (valves) that provide complete protection for the organisms living within them.
Emerging about 525 million years ago, these shells are the most evolved of all six classes and the mollusks are carnivorous. Their shells appear as an "outer hull", but are more commonly internal. There are about 600 of these living species today and, including the giant squid, they may reach lengths of 65 feet.