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Transcript of Phylum Mollusca
Bivalvia - clams, oysters, etc.
Gastropoda - snails, slugs
Class Polyplacophora - the chitons
Clams and mussels formed shells and mantles as they grew
The shell may open slightly to breath, to feed, or to reproduce without exposing its entire body
It's assumed that the first molluscs are of the early Cambrian period
This is about 550 to 580 million years ago
The 5 Classes Of Mollusca
Mollusca's Evolutionary Milestones
Scaphopoda - tusk shells
Cephalopada - Squid, Cuttlefish, Octopus
Includes around 600 extinct species.
Entirely marine species.
Can live up to 7000m deep.
A chiton is flat and elongated.
A chiton has a foot.
Has 8 overlapping shell plates
A chiton's head is small and lacks eyes and tentacles.
Largest group of molluscs
More than 62,000 species
Comprise about 80% of living molluscs
Diverse in size, body, & shell
Occupy all marine habitats
Possess a single, often coiled shell
There are approximately 900 species.
Found in marine regions.
Live in waters greater than 6m deep.
Have four layers, used for identification.
Most live buried in sediment or ice; "safe from predators"
There are 140 species known in the Arctic
Freshwater species are endangered
Stay where first settled; do not migrate
Around 650 to 700 extinct species
Strictly marine species, found in all worlds oceans.
They are the most complex class in phylum mollusca.
Cephalopoda means "head food". They have the most complex brain of the invertebrate.
Has a pair of kidneys
Has etenidial gills (gaseous exchange)
ganglia (cluster of nerves)
paired nerve chords
These types of molluscs crawled on the ocean floor and most likely did not possess a shell
The shell then developed two separate halves that were held together by a ligament.
The shell of a snail is coiled either left or right to form a spiral shape
During their evolution, they developed a dorsal sack which holds the internal organs inside the shell
Torsion is the process of the shell coiling into a spiral
A tusk shell is specially adapted to burrow for food in the ocean ground
It is hollow and open on both ends; one is for the burrowing foot, and the other is for respiration
Cephalopods evolved with a reduction of the shell and a sacrifice of protection, but had improvement in their movability
Cuttlefish and squids have only an internal shell left
Octopuses have no shell left and are at the end of the evolutionary chain
They have several shell plates that overlap like a knight's armour instead of having spikes or scales
These plates improve protection
Chitons have a further development of evolution