The mollusks that have a hinged, two-part shell joined by strong muscles are called bivalves. Clams, Oysters, and Scallops are bivalve mollusks and are a familiar food source. These three animals pull their shells closed by contracting powerful muscles near the hinge. The open these shells they have to relax their muscles.

These animals are well adapted for living in water. For protection, clams burrow deep into the sand by contracting and relaxing their muscular foot. While on the other hand Mussels and Oysters attach themselves with a strong thread or cement to solid surfaces. This method keeps waves and currents from washing them away. Scallops are different. They use their shell to open and close rapidly. As water is forced out, the scallop is rapidly moved in the opposite direction.


This group are the the most specialized and complex group. The animals in this group are squid, octopuses, cuttlefish, and chambered nautiluses. The word cephalopod means "head-footed" and describes the body structure of the invertebrates. Cuttlefish, like most cephalopods, have a large and well-developed head.  Their foot is divided into may tentacles with strong suction cups or hooks for capturing prey.  All cephalopods are predators. they feed on fish, crustaceans, worms, and other mollusks.  All cephalopods live in oceans and area adapted for swimming.  

Squid and octopuses have a well-developed nervous system and large eyes which are similar to human eyes. Cephalopods are the only mollusks that have closed circulatory system, which the blood contains food and oxygen that moves through the body in a series of closed vessels. Just like a human's blood that moves through their blood vessels.  Squids and other cephalopods have a water-filled cavity between an outer muscular covering and its internal organs. 


The largest group of the mollusks are called the gastropods includes snails, conchs, abalones, whelks, sea slugs, and garden slugs. Conchs are sometimes called univalves. Every animal in the group has one shell except for slugs because they don't have a shell. These animals use a "radula" which is a tonguelike organ with rows of teeth to obtain food. The radula works like a file to scrape and tear food materials. That is why snails are helpful to have in an aquarium. They scrape off the algae on the walls and keep the tank clean.

Slugs and many snails are adapted to life on land.  they move by rhythmic contractions of the muscular foot.  The glands in the foot secrete a layer of mucus on which slide on.  Slugs and snails are most active at night or on cloudy days when they can avoid the Sun.  Slugs don't have shells, but they're protected by a layer of mucus around their body, so they have to live in wet and moist areas.  Slugs and land snails damage plants as they eat leaves and stems.