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Mollusca Classes

The Phylum Mollusca can be further categorized into classes.

Amphineura (Chitons)

Chitons have a large muscular foot on their ventral side (underside). Their shell has tiny spots that can detect light.
Their shell is made up of 8 segments. They are the most primitive mollusks. 

Scaphopoda (Tusk Shells)

Tusk Shells have a large muscular foot at the end of their head.

Pelecypoda (Bivalves)

The bivalves include clams, scallops, mussels, and oysters. Bivalves have two shells that are connected by a hinge.

Bivalves are filter feeders. Some bivalves move and dig by using their foot, which can extend out of their shell. Scallops are unique because they can swim by rapidly opening and closing their shells.

Gastropoda (Gastropods)

The gastropods include snails, slugs, and many other single-shelled mollusks. Sea slugs can be very colorful. Sea slugs include Nudibranches and Sea Hares. Terrestrial snails and slugs are the only mollusks that have evolved to live on land.

Gastropods either have one shell (like snails) or no shell (like slugs).

A Word About Shells

Pretty much every shell you see on the beach came from a mollusk. Although shells come in many varieties, they are all made of the same mineral: calcium carbonate. This is the same thing coral skeletons are made of. 

Chiton shells, Tusk shells, and bivalve shells are easily recognizable. If the shell does not belong to one of the previously mentioned groups, then it is definitely a gastropod shell. For example, the famous conch shell belongs to the Conch, a gastropod.

Bivalve shells are flat on the inside. Bivalves have two shells that open and close together. Gastropod shells are usually curved on the inside. Gastropods have one shell, and they have most of their body curled up inside their shell.

Cephalopoda (Cephalopods)

The cephalopods include the squids, octopuses, cuttlefish, and nautiluses. Ammonites are an extinct group of cephalopods. 

Squids have a rigid internal structure called a “pen”, which evolved from a shell. Cuttlefish have a large bone called a cuttlebone, which also evolved from a shell. Some octopuses have no internal shell. This allows them to stretch and fit in very tight spaces. Other octopuses have an internal shell, and as a result, they are not as flexible.

Nautiluses are the only cephalopods with an external shell. The Ram’s Horn squid has a coiled internal shell, unlike other squids.

The Vampire squid is actually not a squid or an octopus. The Vampire squid is in its own unique category.

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