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Molluscs
Zoe Wong
6s
27
Classification:
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Phylum: Mollusca
The Phylum Mollusca contains the following
subgroups:
Class: Aplacophora (solenogasters, deep-sea
wormlike creatures)
Class: Bivalvia (clams, oysters, scallops, mussels)
Class: Cephalopoda (squid, nautilus, cuttlefish)
Class: Gastropoda (snails, slugs, limpets, sea hares)
Class: Monoplacophora (deep-sea limpet-like
creatures)
Class: Polyplacophora (chitons)
Class: Scaphopoda (tusk shells)
Class
Example
Gastropod
chiton
limpet
periwinkle
snail
Bivalve
mussel
oyster
Cephalopod
squid
octopuse
Diversity
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vary from 50,000 to a maximum of 120,000 species
in 2001 estimated about 93,000 named species
Has been estimated that there are about 200,000
living species in total
70,000 fossil species
80% of all known mollusc species are gastropods
Characteristics
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Soft-bodied animals
Distributes in terrestrial and aquatic habitats
An anterior head
A ventral foot
A dorsal visceral mass
Surrounded by a thin fleshy mantle
Sheltered in calcareous shell
Exoskeleton
Carry out respiration by gills
Filter feeder
Body Structure
1 radula
2 mouth
3 shell
4 stomach
5 gonad
6 heart
7 coelom
8 nephridium
9 mantle
10 mantle cavity
11 anus
12 gill
13 foot
14 hemecoel
15 pedal nerve cord
16 gut
17 visceral nerve cord
18 nerve ring
Reproduction
can be sexual
 have both male and female reproductive organs
 mating happens during some particular time and
temperature
 snail stabs the skin of the other snail
 Female snail will then produce eggs internally and then
fertilized by the sperm that has been exchanged
 around 1 month for the snail to lay eggs
 14 days to hatch
 2 years fro the baby snails to become mature
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Evolution - Cephalopods
Advantages of evolution :
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the development of more efficient
swimming
in response to increasing
competition from predatory fish
better locomotion
moving the animal by jet propulsion
colour-changing cells in the skin
Evolution - Gastropods
Reasons for evolution :
 mechanical stability
 defense against
predators
 sexual selection
 climatic selection
Type of gastropods :
semi-slugs
 slugs
 in various other marine
 non-marine gastropods
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Human interaction
pearls of bivalves and some gastropods
 Sea silk
 Tyrian purple dye
 Food
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Sources
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Wikipedia
Answer.com
Andrewgray
Yahoo!
Advanced-level Biology for HK