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Phylum
Mollusca
Phyllum
Mollusca is one of the most diverse phylums around. There are not too many
species; in fact, not one species has each and every mollusk
characteristic. This diversity makes Phylum Mollusca one of the most
interesting and heavily-studied groups, and one of the most beautiful. The
intricate shells and beautiful colors make Mollusks a favorite in fish tanks
around the world, and avid shell collectors are about as numerous as the species
within the Phylum. Characteristics that might be found within the Phylum
include a muscular foot and radula -- a rasping, spearing, or boring tongue --
both found exclusively in Mollusks, a mantle and mantle cavity, a shell, an open
circulatory system, and an advanced nervous system.
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Class
Bivalvia
(Clams,
mussels, oysters, scallops and shipworms)
Class
Bivalvia is a little less diverse than the other main Mollusk groups, but is no
less interesting. The only bivalves that do not have two shells that can
completely cover the soft body of the animal are the shipworms, which use the
two valves that earned them their bivalve position to help burrow into wood, particularly
peers and boats. The shells that are a characteristic of this class are
made of nacre, which is also the pretty stuff that pearls and buttons are made
of. To move around, bivalves extend their muscular foot between their
shells at their posterior end. It is used as an anchor to pull the animal
forward or to help it burrow deep into the sand. Scallops can move my
smacking their dual valves together, propelling them backwards from one place to
another. There are many bivalves on the endangered species list, particularly
in the Midwest region of the United States, including Illinois. Zebra
Mussels, an introduced species since 1985, and the rerouting of lakes and rivers
are two main causes of this problem.
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Class
Gastropoda
(Snails,
slugs, conchs, whelks, limpets, periwinkles, sea hares, sea slugs and sea
butterflies).
Class
Gastropoda is the largest class of mollusks, with 40,000 living species and
15,000 extinct species. This class is very diverse, with members in
marine, freshwater and terrestrial environments. Even within each of these
environments, there is little limitation on where to find a Gastropod. Because
of their soft bodies, the only requirement for their terrain is moisture.
Many of these animals posses a shell to protect these soft bodies, which is
bilaterally symmetrical and may or may not be coiled. Size is not limited
within this class either; fossil records have reached up to 2 meters in length,
whereas some living species are microscopic, too small to see with the naked eye.
The feeding habits of the Gastropods all include the use of the radula.
Most are herbivores, but the variety of their diet is as vast as the number of
species within the class.
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Class
Polyplacophora
(Chitons)
Chitons
are a small, soft type of animal, with a hard dorsal covering composed of a
series of overlapping plates. All chitons are marine animals, living
mostly in the intertidal portion of the ocean among the rocky terrain, though a
few species can be found near the ocean's bottom. They usually can be
found clinging fast to the rocks among which they dwell, and when pried from
their semi-permanent anchor, they can roll up into a tight ball, protecting
their soft underside. Chiton feed on algae and other small organisms
living on the rocks which they can scrape up with their radula, the
characteristic tongue of most mollusks. As far as appearance goes, chitons
usually resemble the rocks on which they live, giving them a camouflage type of
protection.
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Class
Cephalopoda
(Squid,
octopuses, cuttlefish, devilfish and nautiluses)
Each
species of class Cephalopoda is marine, and all are predators. The only
type of Cephalopod that has a visible shell are the nautiluses, though
cuttlefish do have a small shell enclosed by their mantle. The remnants of
a shell appears as the pen in squid. To move, Cephalopods force water out
from the mantle cavity, propelling them forwards. All Cephalopods have a
series of arms or tentacles at their posterior end, which are used in capturing
prey, mating and as sensory extensions. Cephalopods are known for their
huge heads, (hence the name Cephalopod, "cephalo" referring to head) and
are now thought to be very intelligent. In addition to their large brain,
most Cephalopods are equipped with highly advanced eyes, the exception being the
nautiluses. They sport relatively simple eyes.
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How
do mollusks make those beautiful patterns on their shells?
To
understand how a shell's pattern is created, you need t know two facts:
Fact
#1: Color is created by special pigment producing cells on the edge of
the animal's "mantle" (the tissue that makes the shell).
Fact
#2: The shell grows only along one edge, so the two-dimensional
pattern is created by a one-dimensional row of cells.
How
to get a specific color pattern.
All
patterns are created by some combination of the following specific processes:
Unpigmented
space (Shell A): White areas indicate places where dormant pigment
cells exist on the mantle edge.
Vertical
Stripes (Shell B): Stripes running perpendicular to the growing edge
are made with a "static" distribution of pigment. That is
cells at a particular place are always producing pigment, those at other
places are always not producing pigment.
Horizontal
stripes (Shell C): Horizontal stripes, or waves, parallel to synchrony
so that light and dark bands are left behind on the surface of the shell as
the shell grows.
Angle
stripes (Shell D): Stripes at an angle to the growing edge are created
by a traveling wave of stimulation for the pigment cells - cells are turned
on and off, and this wave continues down the growing edge. The steeper
the slope of the stripe, the slower the wave.
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