Phylum Mollusca Soft bodied invertebrates Clams Octopus Snails

Phylum Mollusca Soft – bodied invertebrates Clams, Octopus, Snails

Characteristics • 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 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 – 2 nd 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