Phylum mollusca

1. Phylum Mollusca
“Soft Bodied Animals”

2. Phylum Mollusca
•Second largest phylum
•Soft bodied animals, most with calcified shell
•Four main classes:
•Gastropoda, Bivalvia, Polyplacophora, Cephalopoda

3. Characteristics of Phylum Mollusca
•BodyType: Bilateral Symmetry
•Body Organization: Triploblastic
•3 Layers – Endoderm, Mesoderm, Ectoderm
•Body Cavity: Coelom

4. Body Plan
•All molluscs have four basic parts to their body:
Foot –Takes many forms, used for locomotion
Mantle – Outer covering of the body
Visceral Mass – Internal organs
Shell – Lost or reduced in some classes

6. Digestive System:
•Complete – mouth and anus separate
•Specialized feeding structures: Gills, Beak, Radula

8. Circulatory System:
•Open (Gastropoda, Bivalvia, Polyplacophora)
•Heart pumps blood from gills to open space around organs
•Slow moving lifestyle has lower oxygen demands
•Closed (Cephapopoda)
•Heart pumps blood within blood vessels
•Active lifestyle, large size has higher oxygen demand and requires
more efficient circulation

9. Reproduction
Sexual:
Most are dioecious, some hermaphrodites (gastropods)
Internal or external fertilization
Fertilized egg develops into trochophore larva:
Free swimming, planktonic larva that feeds on tiny plants.
Also found in Annelida suggesting a common evolutionary
ancestor
Asexual: Some regeneration

10. Nervous System
Nerves and ganglion
Brain present in cephalopods
Sensitive touch and chemoreceptors
Eyespots for light detection
Image producing eyes found in cephalopods

11. Respiration
Gills exchange of gases in aquatic environments
Terrestrial molluscs have modified gills inside their mantle
cavity – require moist environment.
Excretion
Nephridia & anus

12. Ecological Roles
Molluscs fill a wide variety of ecological niches
Predators, filter feeders, food source
Break down decaying plant matter
Filter feeding bivalves can be an indicator of water quality
Shipworms (not worms) burrow through wood boats and docks
Terrestrial molluscs are agricultural pests