1. Phylum Molluska Over 50,000 species
Coelomate, Protostomes, Bilaterally symmetrical
Mostly marine, although some live in fresh water or land.
Generally soft-bodied with a hard shell
However, squid & octopi have internalized, reduced or lost their shells.
Body Plan
Although phenotypically diverse, mollusks share 3 common features
A muscular foot - usually for movement
A visceral mass - including internal organs
A mantle - envelopes visceral mass and secretes a shell

2. Molusk General Anatomy

3. Class Polyplacophora (chitons)
Marine animals - shells divided into 8 plates
Move using a muscular foot and graze using a scraping mouthpart called a radula

4. Chiton Anatomy

5. Chitons

6. Class Bivalvia Include clams, oysters and mussels
Have shells divided into 2 halves, connected by strong muscles
Has hatchet shaped foot for digging and anchoring
Are typically filter feeders drawing in water end expelling it through a siphon tube
Generally sessile, however, some species can quickly move by rapidly closing it's shell

7. Bivalve Anatomy

8. Bivalves

9. Class Gastropoda Include snails, abalone, and slugs
Largest class of mollusks ~40,000 species
Exhibit embryonic process of torsion
Visceral mass twists 1800 during development
Most species are herbivores, but some have modified radullas for boring into other mollusks
Some species have successfully colonized land

10. Gastropods

11. Giant African Land Snail

12. Class Cephalopoda Unlike other mollusks, these are quick predators
Have beak-like jaws and poison to catch & immobilize prey
Beak is the center of several long tentacles
Foot has been modified to form a siphon which allows most species to move quickly using a jet of water.
Only mollusks with a closed circulatory system (blood is always contained in vessels)
Very highly developed nervous system
Can learn complex behaviors
Octopi have learned to open jars to retrieve prey
May have rudimentary communication
Cuttlefish and octopi may communicate by skin coloration

13. Cephalopod Anatomy

14. Octopus

15. Squid

16. Cuttlefish

17. Cambered Nautilus