MollusksMollusks

BiologyBiology

JonesJones

MollusksMollusks

• 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 MollusksCharacteristics 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.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 ClassesMollusk Classes

• The 3 major classes are :– Bivalvia– Gastropoda– Cephlapoda

Mollusks with 2 shellsMollusks 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.

ClamsClams

• 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 ContinuedClams Characteristics ContinuedSensory nervesSensory 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 clamsMore 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.

ClamsClams

• 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.

ClamsClams

• 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, ScallopsOther 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.

ScallopsScallops

OystersOysters

• 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.

OystersOysters

Mollusks with one ShellMollusks 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.

SnailsSnails

• 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.

SlugsSlugs

• 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 MollusksHead-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

CephalopodsCephalopods

• Cephalopods have a well-developed nervous system with many ganglia and a complex brainCentral 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.

CephalopodsCephalopods

• 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.

OctopusOctopus

• 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.