Chapter 13.1 Mollusks

Characteristics of Mollusks These are soft-bodied invertebrates, have bilateral symmetry, and usually 1 or 2 shells. Most mollusks live in water, but a few live on land. Examples are: snails, clams, and squid.

Body Plan All mollusks have a mantle – this is a thin layer of tissue that covers the body organs. They also usually have gills – organs that exchange carbon dioxide for oxygen in water. The mantle is also important for making the shell or protecting the body. Mollusks have an open circulatory system – this is where the heart moves blood out into the open spaces around the body organs. (they do not have veins) Most mollusks also have a well-developed head with a mouth and sensory organs. Most mollusks also have a foot used for movement.

Classification of Mollusks The first thing when classifying mollusks are whether or not they have a shell. Mollusks that have a shell are then classified by the kind of shell and the kind of foot they have. The 3 most common groups are gastropods, bivalves, and cephalopods.

Gastropods Gastropods include snails and slugs, as well as, conchs, abalones, whelks, and sea slugs. If a gastropod has a shell it only has 1. Gastropodss use a radula – a tonguelike organ with rows of teeth – to get food. The radula can scrape and tear food. Snails and slugs live on land and usually will be active at night. They eat plants. Radula

Bivalves These mollusks have 2 shells. Clams, oysters, and scallops are in this group. They have powerful muscles that can close their shells. They have a muscular foot for movement. They dig into the soil and stay there – most of these animals are filter feeders.

Cephalopods Squid, octopuses, cuttlefish, and chambered nautiluses are all in this group. Cephalopod means head-footed All cephalopods have well developed heads, they do not have a foot, instead they have tentacles. This group of mollusks are predators that feed on fish, crustaceans, worms, and other mollusks. Squid and octopus have well developed nervous systems and large eyes. They have a closed circulatory system – this means that blood moves through closed blood vessels.

Cephalopod Propulsion Cephalopods have the ability to move water out through an opening in near their head. This water propels the cephalopod backwards and allows them to get away quickly.

Value of Mollusks They are food for fish, sea stars, birds, and humans. Some animals, like hermit crabs, use empty mollusk shells as a house. Many mollusk shells are used for jewelry or decoration. Pearls, produced by oysters also are used as jewelry. Land slugs and snails damage plants. Some snails are hosts of parasites that infect humans. Shipworms can cause damage to boats.