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 Mollusca
The phylum mollusca is the second most diverse animal phylum
with over 50,000 living species and mostly.
The mantle produces the shell. The advantage of a shell is
to protect the creature and offer a portable home. It also has
the disadvantage of taking more energy to move around. Whereas
many mollusca have well-developed shells (snails, limpets etc.)
while other have reduced shells (squid and some sea slugs) or
no shell at all (octopuses).
The advantage of having a reduced shell, or no shell at all,
can be the animal's speed and its ability to get into small cracks
and holes because of its size.
Most mollusca have an open circulatory system expect for
the class of cephalapode. This system is complete with a heart,
blood vessels, and respiratory pigments. Gas exchange is via
gills, lungs, or the body surface. Excretion takes place by means
of kidneys that, like the digestive tract, pump waste into the
mantle cavity. The disadvantage of an open circulatory system
is in directing where the blood goes but an advantage is the
organs are always covered by blood where blood empties into sinuses.
Most mollusca's have a radula. This is a toothed tongue-like
organ used for feeding and defense.
The Phylum Mollusca has seven classes:
- Class Polyplacophora - Chitons
- Class Gastropoda - Snails, Slugs Limpets,
Cowries
- Class Bivalvia - Clams, Oysters, Scallops,
Mussels
- Class Cephalopoda - Octopuses, Squids,
Nautiloids
- Class Scaphopoda - Tusk shells
- Class Aplacophora - Lack shells and worm like in shape
- Class Monoplacophera - Limpet like appearance
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