Compare the foot of mollusks with the tube feet of echinoderms.

Mollusks live in fresh water, in marine environment, but also on land.

Echinoderms live only in water. This is the reason why they have developed different systems for moving. Mollusks have only singular muskullus foot for walking and Echinoderms have tube feet which they use for moving, as well for collecting and transporting food to their mouth.