DEFINITION OF GASTROPODS

The largest class of the phylum Mollusca,

presently containing approximately 1,650 genera but ranges

back to the Cambrian. They occur in a variety of environments

including marine water, freshwater, and terrestrial settings.

Most gastropods have a coiled shell and move via a

muscular foot. Snails are probably the most familiar gastropods,

consisting of a helically coiled shell from which the

head and foot protrude.

Fossil and living gastropods are classified by morphology,

with many aspects reflected in the shell. Cambrian gastropods

had shells coiled in one plane, whereas others show

conical forms. Later forms exhibit torsionally coiled bodies,

with the back of the body rotated so that it lies above the

head. Gastropods are classified into three subclasses. The

Prosobranchia are the most common, show full torsion, and

are found in marine, freshwater, and terrestrial varieties.

Opisthobranchia lack shells or have them concealed in the

mantle, and Pulmonata have spiral shells that have been converted

to lungs, since they are terrestrial varieties.

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