1. You may have walked along a beach and collected seashells. The shells were once part of animals called mollusks. A mollusk is an invertebrate with a mantle, a large foot, and usually a shell. Most mollusks live in the ocean. Some live in freshwater or moist soil. Snails, clams, squid, and slugs are all mollusks.
2. Mollusks share characteristics. many All have bilateral symmetry. All have body cavities, or coeloms, and digestive tracts with two openings. Most have a muscular foot that helps them move. Most have heads, and some have tentacles. All mollusks have nervous systems with brains.
3. All mollusks have soft bodies. Many have hard shells or shell-like coverings. All mollusks have mantles, which are layers of tissue that surround the inside organs. In animals with a shell, the mantle produces the shell. The mantle protects the body in mollusks without a shell. Most mollusks have gills. Gills are organs that exchange carbon dioxide for oxygen in water. Snails and slugs that live on land do not have gills. They have a lung-like organ.
4. Mollusks are grouped into three classes Gastropods include snails, slugs, limpets, and conches. Gastro means "stomach." Pod means "foot." A gastropod's foot is located under its body. All gastropods except slugs have a single shell. When danger threatens, these mollusks can pull completely into their shell. Gastropods have tiny teeth that rip up the plants or animals they eat. Some scrape algae off rocks for food.
5. Gastropods have blood transportation systems in which blood moves through vessels and then out into open spaces around the body organs. This is called an open circulatory system. After blood is pumped by the heart, it leaves the vessels. It sloshes around body organs and then moves to the gills. There, oxygen and carbon dioxide are exchanged. Then the blood moves back to the heart.
6. The second class of mollusks is bivalves. These include clams, oysters, mussels, and scallops. Bi means "two." Each of these animals has a two-part shell that works like a hinge. They use powerful muscles to pull their shells closed.
7. Bivalves are well-suited to live in water. Most do not move around much. Clams burrow under sand with their feet. Mussels and oysters attach to a solid surface to prevent waves from washing them away. Scallops move by rapidly opening and closing their shells. As water is forced out, the animals move in the opposite direction.
8. Like gastropods, bivalves have open circulatory systems. Unlike gastropods, they have no definite heads. They do not have teeth. Instead, they filter food particles from water.
9. The third class of mollusks is cephalopods. It includes the squid and nautiluses. The octopus on the front of this card is also a cephalopod. Cephalo means "head." Cephalopods have tentacles around their heads instead of feet. The tentacles help them swim, walk, and capture prey. They all live in the ocean. They move by forcing water out of a siphon in one direction. This causes them to move in the opposite direction. The giant squid is the world's largest invertebrate. It can grow to a length of 18 meters and a weight of 900 kilograms. That's longer than a school bus and probably weighs more than your whole class put together.
10. Cephalopods are the most complex of the mollusks. They have highly developed brains and can learn difficult tasks. The octopus is the most intelligent of the invertebrates. Cephalopods have teeth. Some produce poisons. Unlike the other groups of mollusks, they have closed circulatory systems. Their blood does not leave the blood vessels.