Phylum Mollusca

GENERAL CHARACTERISTICS
 BODY CHARACTERS :
Soft
Unsegmented
Triploblastic
No jointed appendages
 SYMMETRY : Bilaterally symmetrical
 SHELL : Externally mantle secretes a
hard , brittle , calcareous shell .
It protects the soft body .

DIVISION OF BODY – head , mantle ,
visceral mass and foot
 HEAD : It carries mouth ,
eyes and tentacles.
 FOOT : It is ventral , thick
and muscular .
Variously modified for
creeping and seizing .
 MANTLE : It is a thick
muscular fold of body
wall .
 VISCERAL MASS : Contains
all internal organs .

GENERAL CHARACTERISTICS [visceral mass ]
 DIGESTIVE ORGANS :
Alimentary canal is
well developed and coiled .
 RESPIRATION :
Respiration by gills
enclosed in mantle cavity .
 CIRCULATORY SYSTEM :
It is of lacunar type
with dorsal heart & few
blood vessels .
 EXCRETORY ORGANS :
One or two pairs of kidney .
 NERVOUS SYSTEM :
Comprises of paired cerebral ,
plural , pedal and visceral ganglia .
 REPRODUCTION :
Sexes are usually separate
but may be united .
Gonads are usually unpaired .
Fertilisation can be either
external or internal .

The six major mollusc classes
CLASS 1 : MONOPLACOPHORA
CLASS 2 : AMPHINEURA
CLASS 3 : SCAPHOPODA
CLASS 4 : GASTROPODA
CLASS 5 : PELECYPODA
OR
BIVALVIA
CLASS 6 : CEPHALOPODA

• Foot is broad , flat , disc-like with flat creeping sole .
• Shell is cup-shaped , formed of one piece only .
• Head bears tentacles .
• Example : Neopilina .

• These are the most primitive
molluscs with dorsoventrally
flattened body .
• Foot is flat , broad and sole-like .
• Shell is formed of many plates
• Nervous system is primitive .
• Head is reduced , eyes and
tentacles are absent .
• Example : Chiton .

• Body is elongated and cylindrical .
• Foot is conical and is adapted for
creeping and burrowing .
• Shell is univalved , tubular and is
in the form of tusk of an
elephant .
• It is without eyes but with
tentacles .
• Examples : Dentalium ( Tusk shell ) .

• Body is asymmetrical with distinct head
bearing a pair of eyes and tentacles .
• Foot is broad , muscular and flattened .
• Visceral mass is coiled .
• Shell is univalved .
• Anus situated close to mouth .
• Larva bilaterally symmetrical and grows
into asymmetrical adult due to twisting
of visceral mass .
• Example : Helix ( garden snail ) ,
Limax (Slug ) and Aplysia ( Sea hare ) .

• They are burrowing molluscs .
• Body is laterally compressed .
• Eyes and tentacles absent .
• Shell consists of two valves .
• Example : Clams , oysters and
mussels .

CLASS 6 : CEPHALOPODA
 Body is bilaterally symmetrical .
 Head is large with mouth and
a pair of eyes .
 Visceral mass is well-developed .
 Foot is modified into tentacles
attached to the head .
 Example : Loligo (Squid) ,
Octopus (Devilfish) .

o Neopilina
o Chiton ( Sea Mouse )
o Dentalium ( Tusk shell )
o Pila ( Apple Snail )
o Unio ( Freshwater Mussel )
o Octopus ( Devilfish )
o Sepia ( Cuttlefish )
o Loligo ( Squid )

Phylum Mollusca

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Editor's Notes