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Phylum - Mollusca
(Gastropods, Cephalopods, & Bivalvia)

The mollusks are the large and diverse phylum Mollusca, which includes a variety of familiar animals well known for their decorative shells or as seafood. These range from tiny snails, clams, and abalone to squid, cuttlefish and the octopus (which is considered the most intelligent invertebrate).


Most live in the ocean, but gastropods have adapted to live on land.  Mollusks share common body parts, they are:


  • Foot:  Mollusks use this to move, gastropods secrete mucus which it slides along.

  • Visceral mass:  This contains the gut, gills, and other organs.  It is located in the coelom.

  • Mantle:  It is used to protect the body of mollusks that do not have a shell, such as a squid.  

  • Shell:  The shell protects mollusks from predators and keeps land dwelling mollusks from drying out.  


There are some 112,000 species within this phylum.  The giant squid, which until recently had not been observed alive in its adult form, is the largest invertebrate; although it is possible that the colossal squid is even larger.

Circulatory System of Mollusks

Most mollusks have an open circulatory system in which a simple heart pumps blood through blood vessels into spaces in the animal’s body called sinuses.

Cephalopods have a closed circulatory system in which a heart circulates blood through a network of vessels that form a closed loop.  Humans have this kind of circulatory system.

Mollusks have complex ganglia that control breathing, movement, and digestion.  The cephalopods have the most advanced nervous system of all invertebrates.  They have a brain that connects all the ganglia.  They are considered the smartest of all invertebrates.


Giant clam: Photographed by Jan Derk in November 2002 on the Great Barrier Reef, Australia.Octopus: Photographed by Luis Miguel Bugallo Sánchez

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