Mollusks have a thin layer of tissue called a mantle, which covers their internal organs. The mantle also produces the mollusk's shell. Most mollusks move with a muscular structure called a foot. The feet of different kinds of mollusks are adapted for various uses, such as crawling, digging, or catching prey.
Mollusks have bilateral symmetry.
A mollusk's internal organs include a pair of kidneys.
Most water-dwelling mollusks have gills.
Gills are attached to the mantle and have a rich supply of blood vessels.
The gills of most mollusks are covered by tiny, hairlike structures called cilia. The beating movement of those cilia makes water flow over the gills.
Many mollusks have an organ called a radula, which is a flexible ribbon of tiny teeth. Acting like sandpaper, the tiny teeth scrape food from a surface such as a leaf.
Biologists use the arrangement of teeth in the radula to help classify mollusks.
Snails and Their Relatives
Most snails have single, coiled shell, while many slugs have no shell. Gastropods usually creep along on a broad foot. Gastropods get their name, which means "stomach foot", from the fact that most of them have their foot on the same side of their body as their stomach.
Mollusks with Tentacles
Cephalopods capture food with their flexible, muscular tentacles. Sensitive suckers on the tentacles receive sensations of taste as well as touch. A cephalopod doesn't have to touch something to taste it; the suckers respond to chemicals in the water.
Cephalopods have large eyes and excellent vision. They also have the most complex nervous system, including a large brain, of any invertebrate.
All cephalopods live in the ocean, where they swim by jet propulsion. They squeeze a current of water out of the mantle cavity through a tube.