*Clams, oysters, scallops, snails, and squids are all mollusks.
Mollusks: invertebrates with soft, unsegmented bodies that are often protected by a hard outer shell
-Thin layer of tissue called a mantle covers its internal organs, and an called a foot
-The mantle produces the hard shell
-The foot's function varies: crawling, digging, or catching prey
Body Structure
*Bilateral symmetry
*Digestive system with 2 openings
*All internal organs are located under the shell in the mantle
Circulatory System
*Have an open circulatory system
-Blood is not always inside blood vessels
Obtaining Oxygen
*Mollusks that live in water have gills, organs that remove oxygen from the water
-Gills have tiny, hair-like structures called cilia
-Have rich supply of blood vessels
1. The cilia move back and forth, making water flow over the gills
2. Gills remove the oxygen from the water and oxygen moves into the blood
3. Carbon dioxide is released into the water
Snails and Slugs
*Biologists classify mollusks into groups based on their physical characteristics:
-Presence of a shell or not
-Type of shell
-Type of foot
-Type of nervous system
*The 3 major groups of mollusks are:
1. Gastropods: largest group of mollusks
-Snails and slugs
-Live nearly everywhere on Earth
-Live in oceans, on rocky shores, in fresh water, and on land
-Gastropods have a single external shell or no shell at all
2. Bivalves

3. Cephalopods
Obtaining Food
*Some gastropods are herbivores: animals that eat only plants
*Some are scavengers: eat only dead or decaying animals
*Some are carnivores: animals that eat only other animals
*ALL gastropods eat using a radula, a flexible ribbon of tiny teeth, to obtain food
-Herbivores use the radula like sandpaper to tear through plant tissues
-Carnivores use the radula as a drill and scrape up body tissue
Movement
*Moves by creeping along on a broad foot
-Foot will ooze a carpet of slippery mucus, which helps it move
Two-Shelled Mollusks
*The second largest group of mollusks is bivalves
-Oysters, clams, scallops, and mussels
*Bivalves are mollusks that have 2 shells held together by hinges and strong muscles
-Found in all kinds of watery environments
Obtaining Food
*Filter feeders that strain tiny organisms from water
1. Capture food as water flows over their gills
2. Food particles stick to mucus that covers the gills
3. The cilia on the gills then move the food particles onto the mouth
4. Most bivalves are omnivores: animals that eat both plants and animals
Movement
*Don't move quickly
*Larvae of most bivalves float or swim through the water
*Adult bivalves stay in one place or use their foot to move very slowly
Ex: oysters and mussels attach themselves to rocks, clams move and dig in mud
Protection
*Sometimes sand can get inside the mantle of a bivalve
*The mantle produces a smooth, pearly coat to cover the irritating object
*Over time, pearls are formed

Octopuses and Their Relatives
*Octopuses and squid are cephalopods
-Ocean-dwelling mollusk whose foot is adapted to form tentacles around its mouth
*Not all cephalopods have shells
*Cephalopods are the only mollusks with a closed circulatory system
Obtain Food
*Carnivores
*Captures prey using its muscular tentacles, and then it crushes its prey in a beak and scrapes and cuts the flesh with its radula
*A cephalopod's tentacles contain sensitive suckers
-Receive sensations of taste as well as touch
Nervous System
*Have large eyes and excellent vision
*Have the most complex nervous system of any invertebrate
-Have large brains and can remember things that they learned
Movement
*Swim by jet propulsion
-They squeeze a current of water out of the mantle cavity and through a tube
-Then, like rockets, they shoot off in the opposite direction