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Mollusks Biology Jones Mollusks • • • • Term Mollusk means soft Phylum called Mollusca Over 47,000 species Large range in size from tiny snails of 5 mm to giant squid 66 ft. long • Can live both in sea and fresh water and on land. Characteristics of Mollusks • 3 distinct body parts: head, foot, and visceral mass • Soft body parts are covered by a sheet of tissue called a mantle. • Head contains mouth and sensory organs • Some have a toothed organ called a radula which the mollusk uses to scrape off bits of plant or animal matter that the animal uses for food. Characteristics Cont. • The foot is a muscular structure that clams and some other mollusks use for burrowing into sand. • The visceral mass contains digestive, excretory and mouth. • The mantle covers the visceral mass and secretes the shell. • Gills that function in respiration are located in this mantle cavity in aquatic mollusks. Land forms usually have a mantle that is modified as a respiratory surface. Mollusk Classes • The 3 major classes are : – Bivalvia (means “two shells”) – Gastropoda (means “stomach foot”) – Cephlapoda (means “head foot”) Mollusks with 2 shells • Clams oysters, and scallops belong to a class of mollusks that have 2 shells hinged together. • The animals themselves are referred to as bivalves. Clams • Most live in the sand or mud at the sea bottom , some live in fresh water • Shells are secreted by the edges of the mantle and growth rings are often apparent on the shells. • The shells are held together by ligaments. 2 powerful adductor muscles function to close the shells. Clams Characteristics Continued Sensory nerves • Clams can withdraw completely into their shells. • Clams have no head and no radula, have poorly developed sensory structures. Structures along the edge of the mantle respond to light and touch. • Masses of nerve cells called ganglia are located above the mouth and in the foot. • Ganglia are connected by nerve cords More clams • Obtain both food and O2 from the water that flows through their bodies. • Water enters the clam through the incurrent siphon Cilia move the water across respiratory organs, called gills in the mantle cavity. • O2 diffuses from the waste to the blood and CO2 diffuses from the blood to the water. • The water is then expelled through the excurrent siphon. Clams • Known as filter feeders. • Mucus on the gills traps the food matter, cilia push the food-laden mucus on to the clam’s mouth. • From there the food passes into the stomach. • Undigested food particles leave the clam through the anus. Clams • Open circulatory system • Blood flows through large open spaces or sinuses, rather than through a system of blood vessels. • Clams have a 3 chambered heart that pumps the blood through the clam • Most clams have separate sexes. The sperm and eggs are shed into the water (external fertilization), where fertilization takes place. • The fertilized egg becomes a trochophore larva that settles on the bottom and develops into an adult. Oysters • Cannot move, are sessile. • Early in its live, an oyster permanently attaches its flat lower shell to a hard surface. • The outer shell is rough in texture, while the inner surface of the shell is smooth and often iridescent. • If an irritant such as a grain of sand enters an oyster shell, the oyster protects itself by covering the foreign matter with several layers of shell material. Called a pearl. Oysters Mollusks with one Shell • Largest class called Gastropoda, a name means “belly(stomach)footed” • 37,500 species include snails and slugs that live in water and on land. • Most are univalves, only one shell. • Shell is usually coiled, slugs have no shell. Slugs • Can survive without shells because they live in moist environments. • Like land snails, slugs that live on land respire through blood vessels in the mantle cavity. • Sea slugs respire through gills. Head-Foot Mollusks • Class Cephalopoda includes the more advanced of all mollusks—the squid, octopus, cuttlefish, and nautilus • Cephalopod mans “head-foot and reflects the fact that the head and foot of these mollusks is fused during development. • Well-developed head bears a pair of complex eyes and the foot is divided into tentacles. • Only the nautilus has an outer shell. • The octopus has no shell at all Cephalopods • Cephalopods have a well-developed nervous system with many ganglia and a complex brain Central mouth has jaws and a radula and is surrounded by arms or tentacles. The nautilus has over 90 tentacles, the octopus has eight, and the squid has 8 tentacle-like arms and 2 long tentacles. • Powerful suckers on the arms and tentacles of most cephalopods aid in grasping prey. Some cephalopods, such as the giant squid, have suckers that are armed with hooks. Cephalopods • All are marine animals and live at all depths. • Cephalopods are predators—that is they kil and eat other animals, such as fishes, crabs, and bivalves. Octopus • The octopus is highly specialized for its predatory way of life. • Moves by jet propulsion forcibly contracting the muscles of its mantle and closing the mantle cavity, the octopus squirts out a jet of water through its siphon and speeds off in the opposite direction.