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Phylum Mollusca
Soft – bodied invertebrates
Clams, Octopus, Snails
• Coelomates
• Trochophore larva (aquatic molluscs)
• Divided into head – foot and visceral mass
– Visceral mass contains the heart 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
• 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
• Sessile (foot)
• Filter – feeders
• Nervous system
– 3 pr of ganglia (mouth, digestive system, foot)
• 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
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
• 10 tentacles
• Internal shell
• 8 tentacles
Chambered Nautiluses
• Retains external shell
– Coiled and divided into series of gas – filled
– Soft – body moves forward as the organism grows
– Buoyant