Transcript GASTROPODS Copy diagrams in Black page 53.

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CLASS: Gastropoda
As a class they are long
lived, some appeared in the
Cambrian and at the present
day they are the most
abundant molluscs.
They occupied a number of
life modes, most lived in
water: typically shallow
marine areas but they can
also live in fresh water and
some forms survive on dry
land.
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Modern day forms
include:
Marine:
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Limpets
winkles.
Dry land:
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Snails
slugs (shell less).
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Copy diagrams in
Black page 53.
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The most striking feature is
the coiled shell, forms
because the internal organs
are twisted.
The soft body parts are lined
throughout the shell and
sections could be extended at
will: Head, foot and siphons.
The HEAD extended out of the
ANTERIOR END and a FOOT
extended out of the majority
of the shell APERTURE.
The head consists of
TENTACLES for sensing and
below this is the MOUTH.
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The foot is a muscular organ,
which allows movement.
Terrestrial forms secrete
mucus to aid their movement.
Other soft body parts are
concentrated in the anterior
end where the shell is often at
it's widest.
The ANUS is also at the
anterior end.
Water living forms contain
GILLS and some have a
SIPHON that can be
extended.
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It has one valve = UNIVALVE
and is coiled vertically and
usually spirals to the right
(dextral).
The shell is mostly made of
calcite or aragonite.
In simple terms the shell is a
conical tube closed at the
pointed end (apex).
This end is also called the
POSTERIOR.
The shell is secreted by the
MANTLE and grows along the
aperture.
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Each completed coil is called
a WHORL.
The line along which the
whorls meet is called the
SUTURE.
The LAST WHORL is called
just that the rest are the
SPIRE.
The spire may be high,
pointed with many whorls or
short with a few whorls.
Sometimes the shell can be
flattened giving a planispiral
form (similar to ammonites).
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The size of the last whorl
varies sometimes being
slightly bigger than the
previous one but sometimes
it is much larger.
A SIPHONAL CANAL may
extend the aperture at the
anterior end.
This varies in length and is
used to support the siphon
(that takes in water).
Sometimes the INNER LIP of
the aperture can appear
thickened and almost folded
back as it grew = CALUS.
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In some genera a type of lid
mechanism can be used to shut
off the aperture when the soft
parts were withdrawn called the
OPERCULUM.
The SHELL ORNAMENT varies:
smooth, fine, coarse ribs,
tubercles or sometimes spines.
Because of the coiling muscle
scars are not usually visible.
Most genera coil right handed
(dextral) and therefore the
aperture is on the right. Only a
very few are sinistral.
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See reef video from Discovery.
Generally the marine gastropods
living in the shallowest water
had the thickest shells (littoral
zone) e.g. the cap shaped
limpets.
Gastropods with an aperture
with siphonal canal were usually
carnivores and lived on soft
sediment (e.g. ate bivalves and
other gastropods).
The gastropods without a
siphonal canal were generally
herbivores.
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They did not burrow and
therefore lived on hard
surfaces.
They crawled when
necessary.
The freshwater forms
generally had thinner shells.