In the formal science of naming called taxonomy, mollusks are classified into ten classes: Aplacophora, Caudofoveata, Polyplacophora, Monoplacophora, Gastropoda, Cephalopoda, Bivalvia, Scaphopoda, Rostroconchia, and Helcionelloida. However, only the most familiar organisms will be discussed here. These organisms are members of the classes Gastropoda, Cephalopoda and Bivalvia.
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- Gastropoda: Members of this class are called gastropods. About 70-80% of all mollusk species are gastropods. The term Gastropoda means “stomach-foot” or “belly-footed”. The foot of gastropods secretes mucus for movement lubrication. Examples of gastropods with shells include the limpets, abalones and snails. You can download our snail coloring sheet worksheet below. Do not assume though that all gastropods have shells. Examples of gastropods which lack shells include slugs and nudibranchs.
- Bivalvia: Members of this group are called bivalves because they have two shells. Many bivalves serve as food sources. The foot of bivalves is used either for burrowing or for attaching to wood or other material. Bivalves do not have radulae. This is the reason why they filter food particles from water instead of rasping from surfaces. One process that is unique to bivalves, particularly to clams, mussels and oysters, is pearl formation. When a grain of sand or some foreign body gets into the mantle of these bivalves, the bivalves secrete mother-of-pearl or “nacre” which coats the irritants. Since this is the same material that forms the inner lining of their shells, coating the foreign body makes it less irritating to the bivalves. This process is currently utilized by humans in order to stimulate oysters to produce high-quality pearls.
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