Molluscs evolved in the sea and most molluscs are still marine. Some gastropods and bivalves inhabit freshwater. A few gastropods (slugs & snails) are terrestrial.
The study of similarities and differences in early development of animals is comparative embryology. Triloblastic: three layers develop into species (the rest of the animals we study will be).
As food – mussels, clams, oysters, abalone, calamari (squid), octopus, escargot (snails), etc. Pearls – formed in oysters and clams. Shiny inner layer of some shells used to make buttons.
Shipworms – burrow through wood, including docks & ships. Terrestrial snails and slugs damage garden plants. Molluscs serve as an intermediate host for many parasites. Zebra mussels – accidentally introduced into the Great Lakes and reeking havoc with the ecosystem.
All molluscs have a similar body plan with two main parts: A head - muscular foot used for locomotion and sensory perception. A visceral mass – containing digestive, circulatory, respiratory and reproductive organs. Above that is the mantle – houses the gills and in some secretes a protective shell over the visceral mass. Most molluscs have separate sexes with gonads located in the visceral mass.
Most molluscs have well developed head ends with sensory structures including photosensory receptors that may be simple light detectors or complex eyes (cephalopods).
The radula is a rasping, protrusible feeding structure found in most molluscs (not bivalves). Ribbon-like membrane with rows of tiny teeth.
In some gastropods, an operculum is found on the dorsal, posterior margin of the foot and when the foot is drawn into the shell, seals the opening.
When present, the calcareous shell is secreted by the mantle and is lined by it. It has 3 layers:
The space between the mantle and the visceral mass is called the mantle cavity . The respiratory organs (gills or lungs) are generally housed here.
Many molluscs have an open circulatory system with a pumping heart, blood vessels and blood sinuses. Most cephalopods have a closed circulatory system with a heart, blood vessels and capillaries.
Greatest number in Gastropoda
Umbo is the oldest part. Will be dissecting in lab. No head needed
Pigment that allows them to change colors is chromatophores. Tiny muscles attached to these pigment cells contract to change the color of the animal. Color is very important in courtship displays. Some are bioluminescent. Male takes a spermatophores (encased sperm packet) in his modified tenetacle (hectocotylus) and deposits the spermatophore near the females oviduct so that the eggs are fertilized as they leave it.
Phylum Mollusca <ul><li>Phylum Mollusca includes snails and slugs, oysters and clams, and octopuses and squids (~100,000 different species). </li></ul>
Comparative Embryology <ul><li>Molluscs have a mesoderm lined body cavity – a coelom . </li></ul><ul><li>Lophotrochozoans </li></ul>
Humans & Molluscs <ul><li>Humans use molluscs in a variety of ways: </li></ul>
Humans & Molluscs <ul><li>A few are pests or introduced nuisances: </li></ul>
Mollusc Life Cycle <ul><li>Most molluscs are dioecious, some are hermaphroditic. </li></ul><ul><li>The life cycle of many molluscs includes a free swimming, ciliated larval stage called a trochophore . </li></ul><ul><ul><li>Similar to annelid larvae. </li></ul></ul>
Mollusc Life Cycle <ul><li>The trochophore larval stage is followed by a free-swimming veliger larva in most species. </li></ul>
Major Mollusc Classes <ul><li>Four major classes of molluscs: </li></ul><ul><ul><li>Class Polyplacophora – the chitons </li></ul></ul><ul><ul><li>Class Gastropoda – snails & slugs </li></ul></ul><ul><ul><li>Class Bivalvia – clams, mussels, oysters </li></ul></ul><ul><ul><li>Class Cephalopoda – octopus & squid </li></ul></ul>