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Gastropods
Class Gastropoda |
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Noble
volute
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| Snails
and slugs are abundant on Chek Jawa. The typical snail has a
large muscular foot supporting a visceral mass (the rest of
the body and internal organs) protected by a shell. Slugs have
lost their shells as adults. |
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'Gatropoda'
means 'stomach foot'. There are about 30,000 species
of gastropods, and are thus the largest group of molluscs.
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Shell Facts: The shell is secreted
by a thin, specialised tissue called the mantle. A shell is made mostly
of calcium carbonate and shell material is added to both the outer
edge as well as existing shell so that a shell gets both bigger and
thicker with age. The outer surface of a shell is usually covered
with a tough protein layer. Pigment cells in the mantle create the
beautiful colours and patterns of the shell. The shell protects a
snail from drying out as well as from predators. They come in a a
wide range of shapes, textures and sizes. Some have spikes to keep
off predators, large lips to protect them as they forage for food.
Slugs are mostly gastropods that have lost their shells. Instead of
shells, these creatures have developed chemical and other defences.
Snail door: The shell opening
may be closed with an operculum (a hard trap door) attached to the
foot. Here is a series of diagrams showing
how a snail uses its operculum to seal off the shell opening.
Other gastropod features: Most
gatropods have a head with a pair of tentacles. An eye is usually
located at the base of each tentatcle. Snails also usually have a
siphon, a tube created out of an extension of the mantle. The snail
can stick the siphon out of the shell to suck water in and sample
the water for chemicals, for example, to find food. In burrowing snails,
the siphon is used to get water water to breathe with. In some snails,
the edge of the shell forms a notch through which the siphon emerges.
This is called the siphonal canal.
In addition to a siphon, carnivorous snails usually also have a proboscis.
This is an extendible tube which contains the radula, mouth and gullet.
The Class Gastropoda may be divided into three subclasses: Prosobranchia,
Opistobranchia, Pulmonata.
| Most
prosobranchs are snails with well developed shells and breathe
through gills. |
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'Prosobranchia'
means 'gill infront'. There are about 18,000 species
of prosobranch, making them the largest of the gastropod
subclasses.
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opistobranchs are slug-like with a reduced shell or no shell
at all. |
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'Opistobranchia'
means 'gill behind'. There are only about 2,000 species
of opistobranchs.
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They include sea
hares, sea slugs and nudibranchs.
Opistobranchs have an additional pair of tentacles called rhinophores,
usually behind the first pair of tentacles.
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the other two classes, pulmonates can breathe air. |
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'Pulmonata'
means 'lung'. There are about 16,000 species of pulmonates.
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The gills are
reduced or lost and the mantle cavity works like a lung. Few pulmonates,
however, are marine. Most are found in freshwater or on land and include
slugs and land snails. Marine pulmonates include some limpets. |
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Whelk's foot
and operculum
Gong-gong's
eye-on-a-stalk and
knife-like operculum
Nerite's thick shell
and operculum
Cowries
Drill
Nudibranch
Sea hare
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Gastropod Babies:
Most snails (prosobranchs) have separate genders and generally practice
internal fertilisation. Most slugs (opistobranchs and pulmonates)
are simultaneous hermaphrodites although they may act as a male or
female at any one time. When two slugs meet, they typically exchange
sperm. Most gastropods lay eggs in a case, capsule or in gelatinous
strings and masses.
Most marine gastropods undergo metamorphosis and their larvae look
nothing like their adults. In some, free-swimming larvae with a tiny
shell hatch out. Eventually, they settle down and develop into miniatures
of their parents. In others, tiny crawling snails hatch out. Here
is a fascinating photo
of a gastropod larva on Image
Quest 3-D Marine Library
Doing the Twist: In their larval
stages, gastropods undergo a process called torsion in their development
from to an adult. This process twists the body so that the anus moves
directly over the head. Other body modifications usually ensure the
gastropod doesn't dump over its own head. Scientists don't really
agree on whether there is in fact any advantage gained from torsion.
Torsion is believed to be advantageous because it may help a snail
carry its shell and retract headfirst into its shell. The rearrangement
also moves the mantle cavity and gills to the front and thus avoid
these being clogged up by sediments stirred by movement. It also locates
the osphradium (the sensory patch) to the front where it can be more
effective in sensing the gastropod's surroundings. All gastropods
undergo some degree of torsion during development, and gastropods
are the only creatures to do this. Some gastropods reverse the torsion
later on in their development, e.g., slugs, nudibranchs. Here is a
nice diagram explaining torsion
from the Archerd
Shell Collection website. |
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Nerites mating
Sand collar of
a Moon snail |
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