Phylum Mollusca p. 585

Characteristics Means “soft-bodied” 2nd largest phylum of animals Live everywhere 1mm  20 metres Bilateral symmetry Coelomate Body plan: Foot – sensory organs & muscles Mantle – secretes shell Shell (CaCO3) – protection & support Visceral mass – digestive, reproductive, & circulatory organs

Classes include: Gastropoda – most have 1 shell (snails, slugs) Bivalva - 2 shells (clams, oysters, mussels) Cephalopoda - small internal shell (squid, octopus) Polyplacophora – 8 rows of articulating plates (chitons)

Body Systems Digestive System: Mobility: Complete Free-living & parasitic forms Most use radula (tongue-shaped feeding organ) Bivalvia use gills to filter feed Mobility: Bivalvia – mostly sessile Gastropoda – use muscular foot Cephalopoda – muscularized mantle & siphon for jet propulsion

Excretion: Response: Anus for solid waste Nephridia for metabolic wastes Response: Varied depending on class Bivalvia: simple nervous system & sense organs Gastropoda: nerve ring & nerve cords Cephalopoda: very complex nervous system & highly developed sense organs (for example: well developed brain & eyes); can learn Blue-ringed octopus deadliest octopus No more than 8 inches Venom is 10,000 times more powerful than cyanide Enough venom to kill nearly 30 humans within a matter of minutes

Reproduction: Sexual Bivalvia – separate sexes; external fertilization Gastropoda – hermaphrodites; internal fertilization Cephalopoda – separate sexes; courtship; internal fertilization All show similar features during early development For example: trochophore = a larva (see Fig. 27-2 p.586)

Circulation: All have system with heart & blood vessels Cephalopoda - closed Bivalvia & Gastropoda - open (has sinuses)

Respiration: Bivalvia – gills Gastropoda – diffusion Cephalopoda – gills; O2 carried by hemoglobin

Ecological Roles As a food source (squid, mussels, clams, oysters, octopus, snails) Environmental monitor (but toxic if eaten!) Medical research Some molluscs do not develop cancer Cosmetic products Pearls Damage crops/gardens Shipworms destroy wooden ships/docks “termites of the sea”