Monoplacophora

Meaning of name: "possessing one shell".

Most ancient of the molluscan classes, these are small cap-shaped animals with shells similar to the limpets (Gastropoda). They show repetition of body parts, a primitive feature that shows the phylum's ancient link to the annelid worms. Monoplacophorans are thought to be the ancestors of the gastropods, bivalves and cephalopods.

The few remaining species (about 10) in this group are deep sea dwellers.

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Gastropoda ( "Snails & Slugs" )

Meaning of name: "stomach foot" (i.e.. they crawl on their stomach).

This huge group of animals has adapted to most habitats on earth - land, fresh and saltwater, from the tops of trees to the deepest ocean. Gastropods evolved greater cephalization over their ancestors, with well-defined eyes and tentacles, as well as an asymmetrical spiral shell through a process called "torsion". As with their varied habitats, this group's feeding habits also vary, from rasping algae-feeders to the highly toxic cone snails that harpoon their prey.

With over 75,000 species, the variation in shape and shell pattern in the Gastropoda is enormous.

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Bivalvia ( "Clams and their relatives" )

Meaning of name: "two valves or shells".

This group us generally adapted for a sessile or burrowing lifestyle. They are laterally compressed, having a shell on each side of the body. The burrowers have a spade-shaped foot with which they dig. Most species filter-feed, collecting microscopic food on their gills.

There are about 10,000 living species in both fresh and salt water.

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Scaphopoda ( "Tusk or Tooth Shells" )

Meaning of name: "hollow or boat-shaped foot"

Scaphopods, as their common name implies, possess highly elongate shells, which are adapted for life submerged in the ocean bottom with just their shell tip extended. The head and foot extend from the wider bottom end of the shell, while the smaller hole at the top end is used for water current exchange. They feed on forams and other micro-organisms using specialized tentacles (captactula).

These shells were used extensively as money by North American aboriginal peoples.

The 350 or so species are all very similar and thus, difficult to identify.

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Cephalopoda ( "Squids, Nautilus, Octopouses" )

Meaning of name: "head-foot".

This fantastically adapted group of animals has evolved from the slow bottom-crawling ancestors to be fast moving and intelligent predators. Cephalopods first evolved gas-filled shells (Nautilus, Ammonites) to aid in buoyancy, then gradually lost the shell entirely. The crawling ancestral foot has turned into powerful tentacles surrounding the parrot-like beak; locomotion is via jet propulsion now. The increase in intelligence is an adaptation for predation, Octopus being the most intelligent of the invertebrates.

There are roughly 650 living species.

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Polyplacophora ( "Chitons" )

Meaning of name: "possessing many shells".

Chitons are distinguished by having a row of eight interlocking chevron-shaped shell plates extend the length of the body. Like the gastropod limpets and abalone, these animals are adapted for adhering tightly to rocks. Broad and flat, with a large foot and a surrounding protective girdle, the overall effect is that of an impenetrable suction cup when they are alarmed. Both tentacles and eyes are absent. Chitons feed on algae and other micro-organisms that they scrape of the rocks with their tongues.

There are about 600 living species.

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Aplacophora ( "Solenogasters" )

Meaning of name: "possessing no shell".

These worm-shaped creatures have no shell, mantle, or foot, and as such hardly resemble molluscs at all. To what extent aplacophorans are specialized or primitive is unknown.

The 250 species live either in muddy bottoms or on hydroids and corals.

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