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 Gastropoda Cephlapoda

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.

Other Bivalves, Scallops Scallops live in all oceans, mainly in shallow waters but also in ocean depths. Range in diameter from 2.5 cm to 15 cm. Have a fan-shaped shell that may be smooth or sculptured. Found in many colors, purples, reds, oranges, yellow, or white. Have a single large adductor muscle. Eyes are also located along the edge of the mantle. One scallop may have as many as 100 eyes. Scallops propel themselves by opening and closing their valves and expelling water in bursts.

Scallops

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.

Snails Most snails are less thatn 2.5 cm Australian sea snail sometimes grows more that 2 ft in length. Largest on land is the giant African snail, which grows a shell 8 inches in length.

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.