Their basic body plan includes a foot for mobility; a mantle that secretes a shell and a radula for eating. Molluscs today show many variations on this original body plan. An abalone escapes a sea star predator using its foot; and a moon snail hunts a cockle using its foot as a weapon. The radula has even evolved in an astonishing variety of ways to serve as a tool for feeding. The ancient nautilus was the first mollusc to leave the sea floor. Their ancestors evolved a way to swim using jet propulsion and evolved a way to maintain buoyancy. The evolution of speedy fishes drove the next step in the survival game: speed and loss of the shell, as seen in squid. Octopuses returned to live on the bottom and evolved intelligence and an ability to instantly camouflage itself in order to survive.
Molluscs: The Survival Game
Next Generation Science Standards for this Video
The basic molluscan body plan has been evolutionarily extremely versatile. A muscular foot for crawling and a hard shell secreted by a mantle to protect its soft body parts are successful designs for bottom dwellers. In other molluscs that same foot and mantle have developed structures that function well for swimming.
The body plan animation visualizes the complex mollusc structures and how they function:
- Muscular foot
- Sheet of tissue called a mantle
- Calcium shell present in most