Kingdom - Animalia

Phylum - Mollusca






Mollusks have:

  • a mantle covering the foot and visceral mass.
  • reduced coelom (body cavity) containing the kidneys, gonads and pericardium.
  • a through gut with mouth and anus.
  • a nervous system
  • an open circulatory system with a heart and aorta. 
  • ctenidial gills.
  • a radula - "tongue" with thousands of teeth that can grab onto food

Mollusks are often covered by a hard exoskeleton secreted by the mantle, though slugs, octopuses, squids and cuttlefish have lost their mantle and shell.


Malacology is the study of Mollusks. Malacology differs from Conchology which is the study of shells only. There are over 100,000 described living mollusk species and an estimated 100,000 other species are undescribed and await formal description.

For more information, see the Intro to Mollusca


Basic Mollusca Taxonomy

Bivalvia - Clamsmussel.png

Includes: clams, oysters, mussels, scallops and cockles.

In Illinois:

Bivalves have:

  • Two shell halves connected by a hinge and "teeth"
  • Shell composed of calcium carbonate secreted by mantle
  • No head region or radula
  • Separate males and females
  • Filter feed through siphons
  • Ctenidia (gills) for breathing and feeding
  • A foot to dig into substrate or move along
  • Larval stage (glochidia) are parasites on fish

Gastropoda - Snails and Slugsgastropod.png

Includes: snails, slugs, nudibranchs, whelks, conchs and abalone.

In Illinois:

Gastropods have:

  • a coiled shell - slugs and nudibranchs have lost their shells.
  • a muscular foot secretes mucus on which to glide.
  • a larval stage that undergoes torsion - a 180° twist.
  • asymmetrical bodies.
  • a pair of stalked eyes.



Cephalopoda - Cephalopods

Includes: squid, octopus, cuttlefish and nautiluses

  • Large head attached to tentacles
  • Tentacles are the "foot" divided
  • Lack external shell (except nautilus)
  • Squid and cuttlefish have internal shell
  • Marine - no Illinois species

Aplacophora - Solenogasters

  • No shell, mantle or foot
  • Marine - no Illinois species

Polyplacophora - Chitons

  • Reduced head
  • Flattened foot
  • Eight dorsal valves
  • Marine - no Illinois species

 Monoplacophora - Deep-sea Limpets

  • Single shell
  • Flat foot
  • Marine - no Illinois species

Scaphopoda - Tusk Shells

  • Tubular shell open on both ends
  • No head
  • Marine - no Illinois species



Man has used mollusks for various reasons including money, jewelry, decorations, and as tools. Many species are commercially important. The commercial harvest of freshwater mussels is a multi-million dollar industry in North America where shells are harvested and exported to Asia where they are made into nuclei for insertion into oysters to create cultured pearls. Many species of freshwater mussels and snails are threatened or endangered throughout the world which has sparked renewed interest in their study. If trends are not reversed and stream degradation and loss of habitat continues we will lose many of the interesting and beautiful shells from our nations waters.

INHS Malacologist Kevin S. Cummings has additional information about mollusks.

Some past articles published in INHS Reports: