Mollusca

4 major classes

- Polyplacophora (chitons) - Gastropoda (snails and slugs) - Bivalvia (clams, oysters, and other bivalves) - Cephalopoda (squids, octopuses, cuttlefish, and chambered nautiluses)

class gastropods

• About three-quarters of all living species of molluscs are gastropods •Most gastropods are marine,but many are freshwater and terrestrial species • The most distinctive characteristic of gastropods is torsion, which causes the animal's anus and mantle to end up above its head; torsion is different from the coiling of a shell • Most have a single, spiraled shell • Slugs lack a shell or have a reduced shell

body plan

• All molluscs have a similar body plan with three main parts - Muscular foot - Visceral mass - Mantle • Many molluscs also have a water-filled mantle cavity and feed using a rasplike radula

class bivalves

• Bivalves are marine and include many species of clams, oysters, mussels, and scallops • They have a shell divided into two halves drawn together by adductor muscles • Some bivalves have eyes and sensory tentacles along the edge of their mantle • The mantle cavity of a bivalve contains gills that are used for feeding as well as gas exchange

class cephalopods

• Cephalopods are carnivores with beak-like jaws surrounded by tentacles of their modified foot • Most octopuses creep along the sea floor in search of prey • Squids use their siphon to fire a jet of water, which allows them to swim very quickly • One small group of shelled cephalopods, the nautiluses, survives today • Cephalopods have a closed circulatory system, well- developed sense organs, and a complex brain •Shelledcephalopodscalledammoniteswerecommon but went extinct at the end of the Cretaceous 65.5 million years ago

class chitons

• Chitons are oval-shaped marine animals encased in an armor of eight dorsal plates • They use their foot like a suction cup to grip rock, and their radula to scrape algae off the rock surface

Protecting Freshwater and Terrestrial Molluscs

• Molluscs are the animal group with the largest number of recent extinctions • The most threatened groups are - Freshwater bivalves, including pearl mussels - Terrestrial gastropods, including Pacific island land snails • These molluscs are threatened by habitat loss, pollution, and non-native species

sexual organs/reproduction

• Most molluscs have separate sexes with gonads(ovaries for female and testes for male) located in the visceral mass, but many snails are hermaphrodites • The life cycle of many molluscs includes a ciliated larval stage called a trochophore

ge. characteristics

• Phylum Mollusca includes snails and slugs, oysters and clams, and octopuses and squids • Most molluscs are marine, though some inhabit fresh water and some snails and slugs are terrestrial • Molluscs are soft-bodied animals, but most are protected by a hard shell -triplobastic, bilaterally symmetrical, protostomes -have true coelom reduced to area around heart -open circulatory system(no discrete blood vessels) except for squid and octopus -dioecious (2 separate sexes)