Phylum Mollusca

Soft – bodied invertebratesClams, Octopus, Snails

Characteristics

• Coelomates• Trochophore larva (aquatic molluscs)• Divided into head – foot and visceral mass– Visceral mass contains the hear and organs for

digestion, excretion, and reproduction• Coleom surrounds the heart

• Mantle, epidermal tissue secretes the shell– Calcium carbonate

• Gills– Found within the mantle cavity

• Most are bilaterally symmetrical• Nervous system – Pair of ganglia in head – foot and visceral mass

• Radula

Phylum Mollusca Divided into 7 classes

• Focusing on 3 classes– Gastropoda, one or no shell (snails)– Bivalvia, two shells with hinge (clams)– Cephalopoda, tentacles (octopus)

Snapshot of Mollusca Diversity

Class Gastropoda

• Largest and most diverse group• Single shell• Torsion– Larval development visceral mass twists 180

degrees in relation to the head– Brings anus, gills, and mantle cavity in the front– Allows adult to pull itself into the cavity when

threatened

• Moves by using wave – like motion of foot• Open circulatory system– Hemolymph • Flows into hemocoel (blood cavity)

• Separate Sexes but may be hermaphroditic

Class Bivalvia• Shell is divided into 2 halves (valves) and

connected by a hinge• Close the shell by contracting is adductor

muscles, open by relaxing them• Three layers secreted by mantle– 2nd layer consists of calcium carbonate

Bivalvia• Sessile (foot)• Filter – feeders• Nervous system– 3 pr of ganglia (mouth, digestive system, foot)

Clams

• Bury in mud– Siphons extend past shell– Cillia on the gills pull water in the incurrent sipon– Water/food particles pass over the mucus on the

gills and food becomes trapped• Oxygen is absorbed

– Water exits the clam through the excurrent siphon

Clam Anatomy

• Separate sexes – Gametes are released into water– Trochophore larvae

• Freshwater clams

Class Cephalopoda

• Head – foot• Specialized for free – swimming predatory life• Circle of tentacles from the head• Beak• Advanced nervous system– Lobes with nerve cells– Sensory system

• Closed circulatory system– Rapid movement of materials

• Separate sexes• Egg into juvenile without trochophore larva• Chromatophores

Squid/Cuttlefish

• 10 tentacles• Internal shell

Octopuses• 8 tentacles

Chambered Nautiluses

• Retains external shell– Coiled and divided into series of gas – filled

chambers– Soft – body moves forward as the organism grows– Buoyant