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Mollusks Classification Lesson

by Deanna Maki

​Phylum Mollusca Class

There are a total of 8 Mollusk Classes

• ​Class Polyplacophora

• ​Class Bivalvia

• ​Class Gastropoda

• ​Class Cephalopoda

​Class Polyplacophora

• ​Chitons are mollusks that are flattened dorsoventrally and have a shell made of 8 movable plates

• ​Size: 2-5 cm in length, up to 30 cm

• ​The chiton's radula is reinforced with magnetite (an iron ore)

• ​They cling to rocks with their broad, flat foot

• ​They can roll up like armadillos when threatened

​Class Bivalvia

• ​Bivalves are mollusks that have a two-piece shell

• ​Many bivalves are filter feeders that lack a head-foot region and radula

• ​Bivalves include clams, oysters, mussels, scallops

• ​Some bivalves can form pearls between the mantle and shell

​Pearl Formation

• ​Pearls form when sand, a parasite, or other foreign object becomes lodged between the shell and mantle of specific bivalves

• ​The inner layer of the shell (nacreous layer) is secreted around the irritant

​Class Gastropoda

• ​Gastropods mollusks that typically have a one-piece shell (univalve), but the shell can be absent in some gastropods

• ​Gastropods include snails, slugs, nudibranchs, abalone, limpets,

• ​Most eat by grazing using their radula

​Class Cephalopoda

• ​Cephalopods are active, marine predators

• ​They are the most intelligent of all invertebrates

• ​Cephalopods have a closed circulatory system

• ​Cephalopods include octopus, squid, nautilus, and cuttlefish