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9.8: Types of Mollusks

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What's the world's largest mollusk?

The colossal squid, one of the largest invertebrates, measures 14 feet in length here. Some of these squids are even larger and can grow up to almost 50 feet long! The smallest mollusks are snails that are microscopic in size.

Types of Mollusks

There are approximately 160,000 living species and probably 70,000 extinct species of mollusks. They are typically divided into ten classes, of which two are extinct. The major classes of living mollusks include gastropods, bivalves, and cephalopods (see examples in Figure below).

  1. Gastropods include snails and slugs. They use their feet to crawl. They have a well-developed head.
  2. Bivalves include clams, oysters, and mussels. As their name implies, they have two parts of their shell, which can open and close. Most have no head.
  3. Cephalopods include the octopus. They have a prominent head and a well-developed brain. Typically the foot has been modified into set of arms or tentacles.

(left) An example of a gastropod species, the ostrich foot. (right) A Caribbean reef squid, an example of a cephalopod.


  • bivalve: Class of mollusks that use their foot to attach themselves to rocks or to burrow into mud, i.e. clams.
  • cephalopod: Class of mollusks characterized by a set of arms or tentacles; i.e. octopus.
  • gastropod: Class of mollusks that use their foot to crawl, i.e. snails.


  • Mollusks are divided into ten living classes, including the familiar gastropods, cephalopods, and bivalves.
  • Mollusks live in marine and freshwater habitats, as well as on land.


Use the resources below to answer the questions that follow.

  1. How does the foot compare between abalone, nautilus and a squid?
  2. How does the shell of an abalone differ from the shell of a nautilus? Why is this difference important to the life styles of these two mollusks?
  3. How is the shell of an abalone the same as the shell of a nautilus?
  4. How many hearts does a squid have? How do these hearts help the squid?
  5. How is the mantle of a squid different than the mantle of a nautilus? How does this give the squid and advantage over a nautilus in some situations?


  1. Name five examples of mollusks.
  2. What habitats do marine mollusks live in?

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Date Created:

Nov 29, 2012

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Apr 11, 2014
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