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CEPHALOPODA,
GASTROPODA
&
other Molluscs
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GASTROPODS
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CLASSIFICATION
& GEOLOGIC RANGES
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Phylum
Mollusca (Precambrian-Recent)
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Class
Gastropoda (Cambrian-Recent)
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Subclass
Prosobranchia (Cambrian-Recent)
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Order
Archaeogastropoda
(Cambrian-Recent)
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Order
Mesogastropoda
(Ordovician-Recent)
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Order
Neogastropoda (Cretaceous-Recent)
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Subclass
Opisthobranchia
(Carboniferous-Recent)
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Subclass
Pulmonata (Carboniferous-Recent)
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Class
GASTROPODA
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Gastropods, including such common
forms such as snails, slugs, and whelks, occupy both marine
and non-marine environments. Although many gastropods are
herbivorous grazers, several groups are active carnivores
able to drill through the skeleton of the luckless
victim.
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Most of the gastropods are
classified on the characteristics the gill structures and
other soft-bodied features. Few distinguishing characters of
the univalved shell are used in classification as many are
the result of convergent evolution. Although the differences
in the shell form may be difficult to recognize, different
morphologic groups can generally can be differentiated on
characteristics of ornamentation, shell shape, and aperture.
The shell of many gastropods can either be external
or, less commonly, internal.
The difference can often be deduced by the luster of shell
material and the presence of other features such as
deviations of a structural shell form.
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Gastropods are radulate organisms
with a torted body (e.g., the body is rotated 180û so that
the anus is above the head. Gastropods typically have a
helical coiled univalved shell whose opening (aperture) may
be closed by an operculum.
Another feature which is useful in discriminating among
groups is the structure called the selenizone which can be
expressed as either a series of holes as in Haliotis,
or as a groove along the periphery which is often seen as a
sharp bend in the growth lines of the shell.
Other features such as ribs or the siphonal canal may also
be important.
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Although the shape of gastropods
can be described in terms such as "high-spired",
"low-spired", or "cap-shaped", many of the shape
characteristics can easily be described mathematically by
four parameters S, W, T, and D.
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Figure
1
- Gastropod Coiling Parameters
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Where:
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S
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shape of generating curve (loosely defined as
the ratio between aperature height and width)
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D
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distance of generating curve from the axis of
rotation
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W
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rate of expansion of generating curve
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T
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rate of translation along the coiling axis
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Modified from McRoberts (1998)
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Subclass
Prosbranchia
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The distinguishing shell feature
among prosobranch gastropods is that they are all either
cap-shaped or they are helically coiled.
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Order
Archaeogastropoda. Many
archaeogastropods have an identifiable selenizone (except
for some trochids) in addition to an operculum. The shell of
archaeogastropods can be either internal or external. This
group is exclusively marine. Most are turbinoform,
but others may be high spired,
cap-like as in recent limpets,
or other shapes. Many of the cap-like archaeogastropods have
a small hole in the apex of the cap which is a modified
selenizone. The selenizone of other species of
archaeogastropods may be a series of holes such as in
Haliotis.
As mentioned earlier, the group of involute univalves with a
selenizone called bellerophons, (especially those with
unpaired muscle scars), may be regarded as
Archaeogastropods. Examples of bellerophons are provided.
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Order
Mesogastropoda
Mesogastropods do not have a selenizone. Their shell can be
either internal,
which commonly have a slit like aperture, or external,
in which round or ovid apertures are common, some of which
have a lip. A very common group includes the turritellids,
a high-spired group typical of post-Jurassic. Another common
example of mesogastropods are the slipper shells,
belonging to the genus Crepidula. Other forms, such as the
filter feeder Vermicularia,
often become uncoiled (or vary coiling parameters) during
ontogeny. Of special note is the common low spired
Polynices, who is an active carnivore who drilled holes in
the shells of other molluscs.
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Order
Neogastropoda Neogastropods
do not posses a selenizone, yet they typically have a
siphonal notch or canal and elongated and non-circular, and
commonly slit-like, apertures. Typical examples of this
group occur in the common whelks and others with strong
ornamentation.
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Although other forms such as Conus are also quite
common.
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Subclass
Pulmonata
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Pulmonates can have an external or
internal shell, or the shell may be absent (e.g. terrestrial
slugs). Many pulmonates are terrestrial or live in
lacustrine environments. The pulmonates can typically be
recognized by their often thin shells and distinct shell
morphology which is typically conispiral and rather bulbous.
Furthermore, many pulmonates have a well defined aperture
lip as in Helix.
Here are several other examples.
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Subclass
Opisthobranchia
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Most Opisthobranch gastropods are
marine plankton and lack a mineralized skeleton. Members
that do have a skeleton, including the pteropoda, are
usually small and either cone-like or variable shaped
because their shell is interior. There are no examples of
pteropods and other opisthobranchs in the
laboratory.
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