Phylum Mollusca is the second largest invertebrate phylum, comprising approximately 85,000 extant species, with members characterized by a soft body, bilateral symmetry, and typically a calcareous shell. The phylum is divided into six major classes, including Gastropoda and Cephalopoda, each exhibiting distinct body structures and adaptations. Molluscs possess a variety of traits such as a well-developed digestive system, open circulatory system (except cephalopods), and diverse reproductive strategies.

1. Phylum Mollusca

2. Topic : Phylum Mollusca

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.

8. Cont.....

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.

17. Cont.....
• Also consist on organic matrices.
• Organic matrics play role in Shell formation.