3. Introduction
Second largest phylum of invertebrate.
Member known as molluscs.
Around 85,000 extant species are recognized.
Largest marine phylum.
Divided into 8 or 9 taxonomic classes.
4. Defination
The phylum mollusca is defined as :
• The coelomate animal phylum whose member possess
Head
Foot
Mantle cavity
• Also possess radula and calcareous shell.
5. General Characteristics
Body characters:
• Soft
• Unsegmented
• Triploblastic
• No jointed appendages
Symmetry:
• Bilaterally symmetrical.
Shell:
• Externally mantle secretes hard, brittle, calcareous shell.
• Protects the soft body.
6. Cont.....
Division of body:
• Head: Carries mouth, eyes, tentacles.
• Foot: Ventral, thick and muscular . Modified for
creeping.
• Mantle: Thick muscular fold of body wall.
• Visceral mass: Contains all internal organs.
Digestive organs:
• Alimentary canal is well developed and coiled.
Respiration:
• Respiration by gills enclosed in mantle cavity.
7. Cont.....
Circulatory system:
• Open circulatory system in all except in one class
Cephalopods.
Excretory organs:
• Ome or two pairs of kidney.
Nervous system:
• Comprises of paired cerebrals, plural, pedal and
visceral ganglia.
Reproduction:
• Dioecious or monoecious, gonads with one or two
ducts.
9. Classification
• The phylum mollusca is classified into six major classes:
i. Monoplacophora
ii. Amphineura
iii. Scaphopoda
iv. Gastropoda
v. Bivalvia
vi. Cephalopoda
10. 1. Class Monoplacophora
( Mono=single ; placo=plate ; phora=bearing)
Foot is broad and disc like.
Shell is cup-shaped.
Tentacles present on head.
E.g: Neopilina.
11. 2. Class Amphineura
( Amphi=both ; neura=nerve )
Primitive molluscs with darsoventrally flattened body.
Foot is flat and broad.
Shell is formed of many plates.
Head is reduced so tentacles are absent.
E.g: Chiton.
12. 3. Class Scaphopoda
( Skaphe=boat ; podos=foot)
Body is elongated and cylinderical.
Foot is conical and is adapted for creeping.
Shell is univalved and tubular.
Eyes absent but tentacles are present.
E.g: Dentalium( tusk shell).
13. 4. Class Gastropoda
( Gastro=stomach ; pods=foot )
Asymmetrical body.
Foot is broad, muscular and flattened.
Shell is univalved.
Anus situated close to mouth.
E.g: Helix ( garden snail )
14. 5. Class Bivalvia
Burrowing molluscs.
Body is laterally compressed.
Eyes and tentacles are absent.
Shells consist of two valves.
E.g: Clams, Oysters, Mussels.
15. 6. Class Cephalopoda
( Kephale=head ; pods=foot )
Bilaterally symmetrical.
Head is large with pair of eyes and mouth.
Foot is modified into tentacles.
E.g: Octopus.
16. Molluscs Shell
A wormlike creatures lived in seas known as “Molluscs”.
Typically a calcareous exoskeletons.
Protects and supports soft parts of animals.
Some live on land and in fresh water.
Some of them are Snail, Oyster and Octopus etc.
Characteristics :
• Provide shelter
• Security from predaitors
• Without back bon
• Made up of Calcium carbonate or Chitin.