Phylum Mollusca

Phylum Mollusca

• 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

1. A muscular foot - usually for movement
2. A visceral mass - including internal organs
3. A mantle - envelopes visceral mass and secretes a shell

Classes
There are 8 classes of mollusks, 4 will be covered in this class.
Class Polyplacophora (chitons)

• Marine animals - shells divided into 8 plates
• Move using a muscular foot and graze using a scraping mouthpart called a radula

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

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 herbavores, but some have modified radullas for boring into other mollusks
• Some species have successfully colonized land

Class Cephalopoda

• Unlike other mollusks, these are quick predators
• Have beak-like jaws and poison to catch & immobilize prey
• Beak is located @ 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.